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Lecture 12. Aqueous Equilibria Pt. 1.

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Lecture 12. Aqueous Equilibria Pt. 1.
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UCI Chem 1C General Chemistry (Spring 2013) Lec 12. General Chemistry -- Aqueous Equilibria -- Part 1 Instructor: Ramesh D. Arasasingham, Ph.D. Description: UCI Chem 1C is the third and final quarter of General Chemistry series and covers the following topics: equilibria, aqueous acid-base equilibria, solubility equilibria, oxidation reduction reactions, electrochemistry; kinetics; special topics. Index of Topics: 0:00:00 Review on Strong Acid/Strong Base Titration 0:05:14 Weak Acid and Strong Base Titration 0:10:10 General Reaction 0:13:45 Titration Curves 0:19:04 Calculating pH before any Base is Added 0:31:29 pH at Equivalence Point 0:38:46 pH of a Weak Base 0:44:55 Titration Curve Concept
type Säuren Weak concentration titration basic solutions case period calculations chemical
Acetal species Weak NaCl Bohr titration Arsenic acid Essigsäure chloride gold case man Neutralisation Natriumhydroxid Magnetometer Pitch (resin) molar conjugate Soluble Salts period processes reactor type Eau de Cologne association balance Gases asset chemische Reaktion solutions OH- death end water Natrium Säuren Gamma Iron salts Medicine basic form calculations
Acetal sodium hydride species Weak Petroleum Bohr titration electron donor Essigsäure case man Alfred lead Natriumhydroxid Magnetometer molar Cycloalkane Optische Untersuchungen Stoffmenge mixture level conjugate period processes reactor concentration Wasserscheide Maische Gases asset proton solutions chemische Reaktion buffer OH- water van Säuren Tau protein Gasoline Hab basic form calculations stuff
Acetal sodium hydride Weak titration Essigsäure case man Natriumhydroxid cell molar conjugate period reactor type Holes concentration Gases asset multiple proton chemische Reaktion solutions end OH- water Natrium van Säuren initial Gamma salts basic Converter
titrations Acetal Schweinefleisch Weak Bohr titration Essigsäure case man Magnetometer cell mixture conjugate Medroxyprogesterone period type melting concentration Wasserscheide association Gases asset solutions buffer OH- Säuren coupling Auxin basic atom AdoMet calculations
Paste Säuren initial Weak concentration Bohr Topic basic conjugate case period OH-
let's go ahead and start from the anybody have any questions before it began to Oregon so long you guys remember the last time we started looking at acid-base titration summary said the reason we're looking at acid-base typefaces and class is because it allows us to apply all the principles of acid-base chemistry that we've looked at previously obtained and so on we can use these calculations it illustrates that we know we have a good grasp of all the underlying principles that we've been studying up to this point again and we said we look at acid-base titration is that 3 types of athletes titration your strong acid strong based on strong acid start of weak acid strong based on weekdays strong acid appeared and last time I took the the 1st evidence that was looking at strong acid was a strong base and because remember the example that we took was that the strong base in the direct had .period 1 bowler of the strong base in Europe and would put 100 ml of a strong acid in the flask and I said as I said last time usually viewed carrying this out in the laboratory your goal is to figure out the concentration of the acid in the flask because remember you always part of the solution the unknown concentration in the flask but because this is more of a mental challenged what we want do it we we know the concentrations in the flask as well now would calculate the what the pH would be usually you measure the pH using the pH meter here we do is we the concentration volume in the flask as well and let's see whether we could calculate the pH that you would measure if you put a future leader in there OK and we took 3 scenarios we calculated the pH of the strong acid before you add in various then will look at a case where now we haven't the leaders the base and so the 7th I was calculating the pH after 30 ml of the bees had been added and we said that once the deal is a base had been added to the flask now we had 130 ml so the total volume in the flask after having with 30 ml would be 130 ml In the last that we looked out was what happens after you've passed the equivalence .period and so now we look at the case where we had added 100 and 0 . 5 ml of on base and so the total volume would be 200 . 0 5 ml the last time I wrote this down the 3rd scenario I'd put down the total volume rather they give you the amount these that had been added to list noted that adults and make sure there you write down that demand of this added was 100 . 0 5 milliliters of because what I wrote the last time out only what the problem was I give you the total volume but it did not give you the amount of base that was added so once you add 109 . 0 5 milliliters of various To the 100 ml of acid in the flask give told volume is 200 . 0 5 cases can you make that change new notes and make sure you know down that we add 100 . 0 5 milliliters of the base and the number that idea arguing that problems with the total volume of some of you were confused by that someone clarify that not today Will there
want to looking at the 2nd class and these other ones where are now the look at where we have a weak acid this as strong base we're going to look at a titration where you have a strong base in let's say we have the same base which is . 1 molar sodium hydroxide but now in the last instead of having it see Alvin heavily gasses and a weak acid look at is 100 ml of . 1 molar let's say we're looking at city gasses and this is a weak acid because we know equal to 1 . 7 6 times tend to 5 OK so now going through the same calculations that we did all but wouldn't look at how we OK so before we start doing everything we need to know what kind of reactions of taking place memorably said an acid in the face in large amounts cannot coexist in interview water are right so now what we have considered having a strong acid we know that associates completely we haven't we guess at what we know about the weak acid it does not associate completely in fact only about . 1 % associates of the major part of the week acid almost 99 . 9 Percent remains under associated so the major species in the weak acid will be the UN dissociated assets so if we were to write down the the complete ionic equation for the reaction that takes place we know that in in the last we have a weak acid another weak acid remains under dissociated 99 . 9 per cent of the weak acid remains in this form only about . 0 1 per cent actually associates to give you the conjugate base and hard to undermine appeared so the major species would be the gasses but now I have a strong base to that survive had a strong day it that associates completely so that they instead I add sodium hydroxide kind sodium hydroxide is a strong base it associates completely to give me that so this is what I would have at the beginning before any reaction takes place at times 0 the instant you at the base of the acid at time 0 of these other species that would be there solution even heavier and associated weak acid and associated strong base no we know that an acid and NBA's cannot coexist so we have a weak acid and we have a strong base and we have substantial amounts of gold so the weak acid and strong base cannot coexist with you want what's going to happen this hydroxide strong base for pull off this hydrogen atoms here so that you end up with the conjugated the is Warden an inane plus a right so remember their weak acid we said asses and bases in substantial amounts cannot coexist in reactor neutralize each other you water so you have a large amount of weak acid in blood 1 of hydroxide so the hydroxide will react to weak acid to give you water and its conjugate Bates now in this case you look at the suspect Orion's didn't see that there's only 1 spectator on basis of the suspected Ryan and that's expected rise so you can see that any loss is perspective arrived in the previous example we have to stick it on chloride ions and sodomized spectators but here we have only 1 spectator and which is the sodium and we said if you take a spectator runs out because remember there at the beginning of the end they're not really involved in the reaction to the death spectators out what you end up with is the net finding equation and the net added equations some arises exactly what's going on so you have a weak acid it's going to react with a strong base and we know both of these can coexist with neutralize each other to give you water and so what we end up with is the conjugal plus water so similar but if you want some arrives in the process of the reaction that actually takes place when you have a strong base to weak acid Burnett on its equation summarizes it the weak acid reacts strongly To give you conjugate base and water now this reaction women in tight traditions the reactions have to be clean quantitative and rapid and so this is a reaction that take place completely it takes place cleanly it takes place complete a quantity that means that you're looking at limiting reactant in excess reacted and so on this reaction would take place completely and what you end up with is that since we act and left over a now usually up to this point you would have seen the general reaction or and the general reaction is quite misleading if you look at the general reaction you have to kind of read between the lines to look at the general reaction reactant is the weak acid the base is strong base sodium hydroxide giving you the salt which is this class what I write the what the general reaction doesn't show you is that the strongly affected associates completed as strongly as does the state as awaits we know that once the products of form you end up with water and salt so usually acid-base reaction to result in the formation of water and salt the salt that form is the sodium salt of the acetate and iron but because of the soluble salt you know that associates completely so easily you write it like that we know that it has dissociated completely so this the this is what the reactions look like an overall than it equation is what summarizing exactly what's going on in that solution again so now we want the same thing with calculate the pH when you add certain amounts of sodium hydroxide to its case but the form that I want to show you what the titration crude looks like no to compare it to .period by
looking at the strong asset carousel the medicine at the smaller so this is the strong asset with a strong base this is what we've seen before for hydrochloric acid and In the end we said that hasn't shaker and that's shown here OK now instead of having the acid in the flat last year I have the biggest in the last 11 and the strong acid you can see it now that occurred is is the mirror image of the alright so here to begin with before you add any of the title the pH is 1 if instead in the last year of a strong base you can see now before any tightened is that all you have is . 1 Morla inhale wage . 1 millennial wage will have a pH of 13 because hydroxide tend to make negative wonder therefore had to undermine the 10th it 13 and about the European 213 you see that so if you just look at the pH before you add any of the tight trend you know whether you have a strong acid a strong base in the flask and you can see that this curve is actually related that and so the whole process is not reversed now woman want to looking at it we gas said that now that titration curves looks like this so what you have here is this is the pH and over here the ball be added remember said the shape of the curve it's as strong as strongly as it is an S-shaped curve of signal wirelessly to see the 1st strong acid you see that it is S shape but have a weak acid you have have 2 SSC does 1 guest here in the 2nd year so if you have occurred that has to s in there that means that now you're looking at a weak acid were assessed a strong base all right so that the difference between a plot for a strong acid a weak acid in the back of the depends on this current year depends on the case of the weak acid so here I have a picture that kind of shows Euro what this looks like so we're down here this represents the strong acid that's the typical as Shaker now this is another strong acid over here but now you can see as the assets get weaker and weaker and weaker you can see that that occurred here becomes more pronounced right and saw the end result is that what I want you to remember is that if you take a weak acid like arsenic acid and sodium hydroxide this is what that her looks like OK not very good and if you want to see a picture of what this plot looks like you would collected data in the laboratory this is what it looks like some here what we have is once again on occurred in these actual data points that were collected so you can see that as you add output of sodium hydroxide the dots represent the actual titration and you can see that there is Of course hero analysts 1 like that again if it were like there's that would be a strong acid strong base so this is a titration heard that corresponds to a weak acid versus a strong base now not you look at the equivalent .period so this is the equivalent .period and remember 1 of the things that we said was if you take a strong acids strong at the equivalent .period the solution will be neutral because all the strong acid has been neutralized by all the strong base and all you have been in that solutions salt and water the salt sodium chloride the pH is neutral what the peaches neutral Jennifer Lawrence all you have been there is neutral solutions and therefore the way to tell where the caller's point is you look for Pete 7 and that will tell you the equivalence .period now in this instance the equivalent 4 and will always be great than the solutions can the basic and the reason is that if you look at the next summit equation so here we are you look at this and that and antiquated and you know that the weak acid reacts with hydroxide to give you acetate so can you see that here what we have is the conjugate base of a weak acid at Boquillas .period you have water which is neutral and you have the conjugate base of a weak acid the confidence of a weak acid is it acidic basically neutral it could be basis and therefore the pitch has declared himself 7 to see that so for weak acid strong base titration and equivalents .period the pH will be basing it has to be about 7 and you can see in this plot at around 8 OK so this would be the equal and right here which is about 8 now in this region and you can see that it's almost flat everything that plot the pH ,comma changes in this region are right and the reason that the pH hardly changes in this region is because as you add strong base to acetic acid which of forming his acetate and 9 so in this region and what we have is a weak acid and its
conjugate base whenever you have a mixture of a weak acid and its conjugate based in substantial amounts in approximately equal amounts what is it like a buffer scan see that in that region it's acting like a buffet because you have bolt the weak acid and you have the weak base and because both of them are co-existing that we didn't win act like applicable offer and if it acts like a buffet you can see the pH hardened changes himself that's why we see this flat region here but this is a flat region where the pH hardly changes are right and on here is another example of that flat region where the peace hardly changes OK so already doing is now working as do the same calculations that we did with strong acids and strong bases now will look at it in terms of weak acid strongly OK and so wouldn't on the presumption that we have put on molar sodium hydroxide in there wouldn't have 100 ml of . 1 Boulder city gasoline in the flask so calculating itself once again we have to to go to the whole process a :colon so we start by looking at the 1st scenario where we calculate the pH before any Vegas is added right before any this is added what we have is . 1 molar acidity assets and we know the case of the acid is 1 . 7 6 times 10 to 95 show all of you know how to counter the beautiful we gasses how'd we do that right now but that's the 1st kind of problems that where we have a city gas said you can set up the equilibrium so what we have is we have a city gas said it quest in water giving me the court this is there are said this is the basis this is the proton donors this is the proton accepted so what you end up with is the conjugate base and had mind on new remember that OK and we know Katie is 1 . 7 6 times in the next 5 we say at the beginning of the concentration of acidic gasses .period 1 molar when addressing water 0 0 at the change would be minus X water plus 6 plus 6 and equilibrium we know this is the point 1 minus X we know this will be the acts we know that K equals 1 point 7 6 times and the negative side which equals the concentration of acetate times times the concentration of petroleum mine divided by the concentration of acetic acid which is exquisite "quotation mark work . 1 -minus we can approximate that X squared divided by . 1 kilos the process so you solve this OK and validate approximation and so on and I'm going to go to the whole process because we were problems like this so many times in the United captain the weak but we know the calculated X X gives us the high during my concentration which turns out to be 1 . 3 3 times 10 to the needed 3 more alert and therefore if you calculate the pH of that solution it comes out to be 2 . 8 7 7 so I want to try this at home and make sure you know you know the process case so all we have is a weak acid in the flask and we don't have to appear weak acid wants to know the concentration so that the pH before we added base now and on the 2nd scenario where do you look at what happens when we had 30 ml became the 2nd scenario is woman chemically pH after 30 milliliters of any wage is adding now said In the titration because the volume is constantly changing because we're calculating the pH in the flask and because work incrementally adding sodium hydroxide and because the volume is changing in other words diluting the solutions we need to we can think of a lead concentration because concentration is molds divided by the volume and the the volume changing that means a concentration is changing and therefore In these calculations we always counted the number of malls because the number of most of the acid and does not change as you as the reaction takes place you can figure out the number of Malta the acid that's consumed and then we can figure out the number of molds that's left over and reconfigure the number of malls and would encounter the new concentration again so we're going to use the same strategy that news last time so will start by calculating the initio the number of moles of city gas said so we want to know how many malls of the acid are actually in the flask spawned reaction takes place and that it is . 1 0 0 malls from their times we had 100 ml which is . 1 0 0 litres here would take years and 3 significant figures so that comes out to 1 times 10 to the negative 2 walls of acidic gasses so before any reaction took place this is the number of moles of acetic acid that you have a case likewise would be calculated that in the show the number of mold of hydroxide I'm that we had to this is that the number of roles of hydroxide that's in the 30 ml that you determine now likened where concentration to moles member we had 30 ml of any OH To the flask surrounding calculate the number of malls of hydroxide in the 30 millilitres water waiting to know how Kaplan by the fact that the concentration of times the volume again Sunday at a concentration times volume so I have .period 1 moles per liter of sodium hydroxide times I added 30 ml so that becomes but because of 3 significant figures said because . 0 3 leaders and that gives
me 3 times 10 to the negative 3 walls of hydroxide so what whatever calculated the number of moles of each of the reactants before he reaction took place now to figure after this reaction is completed how much of each component do I have a scenario that likely to write the net on equation what was the name of the equation for this we know that we gasoline so we start we gasses so we have city acid which is the weak acid McCain the other reactant was hydroxide it reacts cleanly rapidly and quantitatively To give Maine the conjugated base which is the acetate and 9 plus 1 case so would establish saying before any reaction takes place when I have is I just calculated that I have 1 times 10 to the negative to of acidic gasses and then I have 3 read times 10 to the negative 3 moles of hydroxide and I had none of that cannot concern about water because water is a solid take so now will let the reaction takes place so what we have is that after reaction occurs no this is a limiting reactant problem you can see that this is excess this is a limiting all right so that means that divides subtracted from that was what I end up with a 7 times 10 1083 malls all of this is consumed and the reason it's consumed is because this is a limiting reactant and this is the excess reactor case some or all of this reacts vacancy on the product side I and with 3 times tend to native 3 malls all right so once this reaction has taken place we know that we have some on reacted to City Council left and we formed the conjugate base so know what I have been advised that we gasses conjugate base how's that is to you have a weak asset and its conjugate base together in approximately equal amounts when you you can act as a buffer it's about 400 the pH of a buffer you can use the about great art so now I know the walls of the city gas I know the walls of its conjugate base provided applied has announced that house about equation I need to know concentrations are right it's a Harlequin With walls to concentration divide by the volume what is my volumes 100 million I hadn't series and now my total volumes was 130 case so now I'm going to go out the new concentration so itself and the started by capping the concentration of the excess reacted that's left over which is the moles divided by the new volume which is 130 leaders of . 1 3 leaders or 130 ml and that comes out to be 5 . 3 8 times 10 to the negative 3 out 10 to negative to Moeller and then and then calculated it's kind of base which is 3 times tend to 93 balls divided by . 1 3 leaders which gives me 2 . 3 1 times 10 to negative to Moeller K so now I all the concentration of the weak acid and it's kind could be it's now I can say pH equals -minus log the concentration of the acid form provided by 5 the concentration of the acetate in Anaheim after the titration is completed so we can assume that that's some new initial and so this is a city gas and so this is full .period 7 5 4 -minus large concentration of the acid is 5 . 3 8 times tend to negative to divided by the concentration of the acetate which is 2 . 3 1 times 10 to the negative 2 and so that gives me the pH of 4 . 3 8 7 OK so now we have countered with future that solution is in the flask can only see that what we're doing here is the same stuff that we've done this for except that now we have a lot more things going on and I want you to be able to understand this at the same level are right there is a lot of detail there and the kind of understand that in this instance what's happening is there's a reaction taking place after that reactions complaint 1 calculate where the species in that solution in the blast and we need to cover the concentration of those and from that we can figure out the peaches no was homework and I ask you guys to calculate what appeared to that solution would be if you've added let's say 45 ml all right we had 30 ml now want to try to calculate what would appear to that solution turned out to be if you have 45 mm all right now with a look at the text scenario which is at the equivalent .period so the next 1 will be be so we looked at after you had 30 now let's look at a pH at the the equivalence .period all right so we need to calculate what the equivalence .period remember what is equivalence .period recalled as equals .period for the stood geometry .period and what is that represent the retirement what happens if equivalence .period equal amounts in other words the number of molds of the acid should equal the number of Malta the base in other words all of the acid has been completely neutralized by the base and usually when you calculate the call applying the underlying principle is that the number of balls in this case the number of moles of acetic acid should equal the number of moles of hydroxide that you have that solution they have both reacted a one-to-one state geometry
because the bounced equated shows us that they react in a one-to-one ratio cell if all of we acid has been neutralized by the strong base then you have to have the same number of multiple and we know that walls as well concentration times volume so on this side we have concentration of acidic gasses times the volume as suggested in over here you have a concentration of hydroxide which is sodium hydroxide times the volume of hydroxide which the volume of sodium dropped that you added a case so at the conference .period 1 a calculated the volume of hydroxide that's there that would be the concentration of acetic acid at times the volume of acetic acid divided by the concentration of hydroxide which is concentration of acetic acid it is .period 1 most believe there times the volume was that we had 100 ml and we know that we had .period 1 moles per liter of base said can you see that the volume of the base should be . 1 ml now sometimes in the laboratory lies remember sometimes in the lab to give you this equation M 1 the 1 equals into to this is the same thing and it is the malaria volume is the volume of acid that you added and an end to the 2 would be the more loudly at the base times the volume of the beast that you had yet to be careful because you can only apply that if the acid in the base react a one-to-one ratio are right so that you have 1 the proton are right and therefore the hydroxide will react without 1 city Proton delivered to one-to-one ratio are right so what are we not at what how much of this I need to add at the close .period you can see that I have to add 100 ml right survive at minute and 100 ml of base I would be at the equivalent .period right so that when it happened th now I ever look at this so pH at the "quotation mark was . it is remember the reaction looking at is the weak acid reacting with the bees to give you the conjugate base plus liquid water a case and we said before reaction we had 1 times 10 to the negative to moles member in the flask when you have 100 million of . 1 molar that's the number of holes of acidic gasses that you had to start about 100 ml times .period 1 now we had because by we have to have 100 ml to contaminate tell me how many malls of hydroxide did we had 100 ml . 1 molar Oneil wage so what will be the number of molds of any away what she Lara it's the same thing you understand that because of the current when the number of malls have to be the same as in the mold of assets have to be the same as the most basic good so you should see that you're the coolest points even the same number of holes should never received that this would be in the to and have 0 right now have 2 reactions to the reaction take place this is the titration To the acid they cannot coexist in reactor cleanly rapidly and quantitatively so the reactors that were completion so now see these are the same amounts of all of this reacts with all of that you wind up with 1 times 10 to the negative to malls of the product summarily said at the earliest .period all the acid has been used up although this has been used up so what are we left in solution at the end of the titration the conjugate base it's now we need to counter the pH of the country could be it's time to get out of the bead of the pH of a weak base I write I the concentration of our rights and they take a number of malls and calculate the concentration and therefore that would be the concentration of the conjugate base which is 1 times 10 to the negative to malls divided by the volume of what is my volume I had 100 ml in the flask what's the volume of waste that I added 100 so what is my total volume 200 became something to divert the total volume which . 2 0 0 meters which gives him . 0 5 0 0 malls per liter said now we go back to working amendment is a conjugate base How do I counted the pH of this practice all you see that it's this it's the salt of the conjugate base of the weak acid acetic acid I know the initial concentration 2nd everybody write down to how you count indicated that solutions this is applying what we did like that the 1st few weeks of classes what's so you're not happy the pH of a weak base what is the starting point where you need to write down when you start with what you need to write down higher up the pH of a weak base although he did really well on the tests of you to know that you have to draw the equilibrium and news I start I see that so so what right down to the equilibrium 1st seller this is city acetate and which is the salt of the conjugate good faith in water giving me the weak said Falwell put here all which might great and this represents KB they give McCain no beginning case I need to convert Katie KB therefore KB equals 1 time Stanton 14 divided by 1 . 7 6 times in the name of 5 which gives me the KBE which is 5 . 6 8 times 10 to negative 10 U.S.
understand I can move really fast because we've done many problems of this type role once again just applying what we learned before can that 1 right the I stressed out with my initial concentration the . 0 5 0 0 I'm not concerned about water I have 0 0 to begin with but I wanted to go with the changes made next plus 6 plus X not concerned about water at
equilibrium I know this will be . 0 5 0 minus X X X so we're looking at key because this is the formation
of hydroxide from the weaker confident basis at 4 kb equals 5 . 6 8 times 10 to negative 10 which the concentration of the acetic acid finds the concentration of hydroxide divided by the concentration of these right so that gives me extra where overlooked . 0 5 0 0 minus X right and as always we know we can approximate there's 2 x squared over . 0 5 0 now did you guys know how to solve this I would like to try this at home you have to check the validity of the approximation you calculate acts after you checked for validity of approximation so on X will come out to be 5 . 3 3 times 10 to negative 6 smaller so we know that the hydroxide and concentration equal X which is 5 . 3 3 Contents tend to of smaller Member we need to cap pH signing to convert that to I during concentration which is kW divided by the hydroxide and concentration which is 1 times 10 to negative 14 divided by by 5 . 3 3 times 10 to the negative 6 more this will be more squared and therefore the work that out it comes out to be 1 . 8 8 times 10 to the negative 9 smaller therefore pH equals 8 . 7 2 7 remember we said adding "quotation mark was coined the pH has to be greater France 7 because that solution is facing our right insult we've figured out what the pH of that solution is at the equivalence .period objective now I need to be able to calculate cell if you look at the 4th scenario you need to be able to calculate the pH after 100 . 0 5 milliliters of India which is added OK now the solution to the US is very similar to the strong asset because once we passed equivalent .period now although after visiting consumed completely and all you have is a strong base are right and so the solution for this part will be exactly the same as a solution for the previous parts because what you have is now at the close .period you about 100 ml of the base now you have an accessible . 0 5 are right so that solution that have a strong base and a week later and astronomers is much stronger than the be base to see them so the major contributions will come from the strong base so you have a point of 5 milliliters of excess hydroxide all you have to do is calculated the number of molten . 0 5 ml of hydroxide divided by the total volume which is 200 . 0 5 and that gives you the concentration of the strong base and because strong associates completely you know the hydroxide and concentration is and so you know the hydroxide and concentration that comes from that extra point you'll find that was not neutralized all right in the capital of the future that solution OK so you should be able to solve these kinds of problems and discussion for next week actions this week that the discussions actually get you to solve some of these problems I need to be able to look at these different types of problems and to be able to solve them again now a couple of things I want point out actually go back to the graph and I would point out some things on this so thin you look at this titration this is the point at which there is no basis added to usually over-the-counter repeated that we gasses OK we know this region where it flattens out is called the buffer region Abkhazia in this region where flat we call that the buffer region because that's the region where it's going to act as a buffer so whenever you have mixtures of the weak acid and its conjugate base is going to act as a buffer again now this is the equivalent .period all right and we know that the coolest .period the pH will be greater than 7 OK no if you take we already calculated that you have had 100 ml to reach the cruelest .period so exactly half of that is called the have equivalents .period variety and that he had equivalent .period which you realize there's itself at the have equivalents .period what to realize is that the concentration of the acidic asset that's left over equals the concentration 3rd C H 3 C O 2 minus tonight as we write that OK because it's not very clear and don't want to rush that so at the time have equivalents .period we know that the concentration of the acidic gasses that's left or when the flaps that excess amount should be equal the concentration of the acetate find so you know that th equals PKK -minus log of the acid for over the base and these identical equal to each other what we know PHE calls PK minus logged 1 which is peaking minus 0 so we know what the half equivalence .period pH city should people picking so you have articles .period you figure out that the cruelest .period 100 ml here happy goers .period that's the perfect buffet yet equal amounts of pork are right and therefore the pH should be equal to so if that's the case you wanted to any calculations and you get a question saying you know the pointedly equalized .period they ask you once appeared to the solution at this point you're not doing that because you know pH equals became and this gasses a begin acetic gasses about 4 . 7 5 so you know that the pH at that point will be 4 . 7 5 effect now I want to finish up by just put going through this question so this is an old question from an older exams are just full of it this morning and I want you guys to take a look at this and put a copy of this on the class website OK so why is that here we are so out of the question consider the titration of a generic weak acid with a strong base that gives the following titration curves so you have already seen this Caribbean it's a weak acid strong base could obtain all their indicating the points that correspond to the following the 1st the equivalent .period so all of you know at the close .period this tiger magic .period has to be there that is so that they could point to the region with maximum offering so you wanna show it kind of shows where this is plateaus out flattens out this is the region of maximum offering somewhere between this and this and that region that PAT equals the where would that you look at the equivalence .period it's a little above 25 are right in 30 years at somewhere between states it's actually summary fear right in the middle so you have to look at this value and divide that exacted by half and that the interview with PET was
and that would be point their take pH depends only on the initial concentration so this would be a subscript OK and so on it depicts depends on only the initial concentration of weak acid where would that 0 before you add in the case that would be the point at which Piech depends only on the conjugate base at the close .period because the cause .period is the only point at which you have exclusively just the constant base last pH dependent only on the amount of excess strong base added so where would that be where the pH depending on the amount of strong base anything past cocoons .period so once you pass this all these points when surpasses anything about that will depend on the concentration of the hydroxide OK so will stop
there today and next time we want next topic the