Lecture 08. Acids and Bases. Pt. 5.
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Lecture 08. Acids and Bases. Pt. 5.

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8

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26

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CC Attribution  ShareAlike 3.0 USA:
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University of California Irvine (UCI)

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2013

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English

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Abstract 
UCI Chem 1C General Chemistry (Spring 2013) Lec 08. General Chemistry  Acids and Bases  Part 5 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 acidbase equilibria, solubility equilibria, oxidation reduction reactions, electrochemistry; kinetics; special topics. Index of Topics: 0:00:00 Putting Salt in Water Review 0:05:50 NaCl Example 0:09:41 Anion Possibilities 0:11:48 Calculating pH of NH4Cl 0:21:10 Acid in Water 0:27:06 Common Ion Effect 0:37:06 Calculating % Dissociation Common Ion Effect

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OK can I have your attention to the of was going to start I want to make 1 announcement and that is that you know that there's a midterm coming up next Wednesday a week from today we have the 1st midterm exam and in preparation for their return to their homes became department tutors will be holding a view on Friday from 5 to 7 I'll write and that information is on the class website during the 1st hour that review I will going there to answer any questions so I so this is a good opportunity you have a questions about please come prepared to ship study materials so when you have a review is you after you've studied Mitchell ran sort of coaching you alright so please come prepared its on Friday from 5 to 7 PM and information is on the class website came down for the will start last time you guys remember last time we talked about our asses these properties of assaults and recent assaults on ionic compounds in other words solved is an ionic compounds and we said you dates also taken the evil soluble in water or insoluble in water the majority of them are solid soluble arrive and you take the soluble salts we said at the moment to put solid dissolved in water it disassociate completely sodium chloride and even to ground water that instead it you makes up water it
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associates completely so I'm sure you will molecularlevel picture of what happens OK so we have sodium chloride and Miss started again I have the role of 1 of
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the coastline right when they have categorized it incorrectly
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it was so stimulative for
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that theory that's the right 1 How OK so I wanted to take a look at this picture in which you see in the beginning is a sodium chloride as a crystal you can see that and ionic structure the young 1 of the green ones the chloride and the great ones are over so you can use cannons always smaller than an alliance in the end solidly packed together is a land and sea you can see the light structures in the What happens is we all want to come out of it you can see the whole structure disintegrating as water molecules Rapid City South around the cat and and so you can see this quarter where you can see the ironic solid said here are the cat eyes and the green ones of chloride and the great ones are the sodium cats and so we can begin with this is where the I'm solid looks like a right away at room temperature so you can see that they're migrating back at their location is the same said they vibrate about a fixed location would you pour water over and you can see now the water molecules are attracted to the chloride and alliance and the sodium can and so as a result of that the whole structure starts disintegrating and falling apart you can see that the cannons in Anza removed from the lattice structure and the whole structure disintegrates now wants to form the sodium chloride solution and this is
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what it looks like so here now after the sort of Florida's is all completely you can see here a chloride and right surrounded by water molecules and then you can see the so of sodium I'm coming up there to sodomize occasionally that kind of against each other and then no go away all right so what's happened is the water molecules and such logic says that in the end every continent and be surrounded by water molecules and separated by great distances because there's so much "quotation mark fund solvent molecules compared To the sodium chloride my on occasion so if you wanted to write down in the fall of an equation what we just saw so only to look at what has really take sodium chloride this is a salt and water we said and disassociate completely To give you any plus plus the minus the quest in reality take in a and you want to represent that had and a sphere remembered in the same period so you compare the size of the cat the and we know that constantly smaller so the smaller ones would be the sodium content and in an instant the larger into the take KhanAcademy's positively charged and we know water molecules uphold so you have dealt a minus Delta like that and so it turns out the reason they dissolve is because the attraction To the water molecules is greater than the attraction for their comrades so in certain chloride solid but holds it together is a presentday attractions for for the moment but in water that traction for the Catalan and I water is greater than the attraction for each other and therefore it is also and so on this structure is represented as any Aplus Equus summoned France's when we states inquest it means that it's surrounded by water molecules likewise you have sealed miners conceal minuses bigger size wise and its negatively charged you know what happens is that the Delta lost in the water molecule it will be attracted to the chloride and I and sell these water molecules will wrap themselves around the and and this is the reason why soluble salts dissolved right now we said can be insoluble as well when you look at insoluble salts it turns out the attractive each other's far greater than the attractive for water and therefore insoluble salts don't dissolve in water it sink to the bottom and call those precipitates rank right now interested in looking at soluble salts and said that soluble salts like sodium chloride can act as an acid base or neutral substance in water and for you to figure out what it could be an asset base of a neutral substance which you have to look at is the cat and that make up that the substance of salt and we have rules that activeduty and we know that at the end I'm part of it is a conjugate base of a strong acid we said it could be neutral Indiana the conjugate base of a weak acid it will be basic in water right those are the only 2 choices you have you take the account I'm part of it we said at the cat turns out to be a Group One metal or group to metal it's going to be neutral so like sodium potassium and so on it's going to be neutral in the contest is the conjugate acid of a week then it will act as a passes are right and so on you look at the table that I you know that most of the acids and bases that I gave you that is on the club's website you can see that in the American Red these conjugate bases are these in red use of the condom bases so these contradictory bases that are highlighted in red other continent faces a strong acids so if you and finally assault is any 1 of those you know it's going to be neutral in water are right it here and I happens to be any 1 of these conjugate bases that I had negative charge case and look for the ones that have made charges look at as rehab is neutral so neutrally can act as a Iowa and writes a leaving that look at everything else that's negatively charged all of these will act as a base in water are right at the counter and the Canada sodium potassium Group 1 medal right charities and solves the designers form Will the basic in what appeared now you talk about acids of Greek vases and you look at these any 1 of those that are positively charged here see this was positively charged that means can act as a cad and therefore in water it will be a severe is 1 example and you go down anything that has a positive charge here this 1 is the period mind and this is a positive charge different you place in water and this will be a weak acid arrest because it this is the conjugate after its base would best and therefore as can likewise finished 4 last is the conjugate acid week base in its 3 therefore this can act as a catalyst all right so long as you have a table in front of you you can identify the ones that will act as a conjugate acids in assault because 1st of all you have to have a chat the acid for it if it has a positive charge can act as a check and in the basic side it has a negative charge it can act as a weak bring solve OK so now only have to do his work some examples of trying to figure out the pH of some salsa tastes of the nextlargest calculating that the amount of high doing a mind of his outside case so here is a question where once again remember what kind of things that we've been developing is that you should be able to calculate the pH of any kind of solution whether you have a weak acid strong acid when you have a week based on days when the you have a polyploidy gasses regardless of any 1 of these examples that we looked at the Schaeuble the calculate with the peaches right so in this example we asked to calculate the pH of . 1 molar ammonium chloride C a starting point if you look at the substance that's given to you you ask yourself or all of that substance a weak acid strong acids we made a strong bid Pollack predicted on the assault are those old examples that we looked at so we'll look at that compound in what would you have and you can rest it's not a case so that the that the salt you know that the obvious and beginning you should be a soluble salt because of its it's not something you can't happen people affected so this is a soluble salts and you know that the soluble solids based integrated water what will happen it's going associate so we know that if I take ammonium chloride I know that the instead is dissolves in water just like that picture that I showed you it's going to dissociate completely in
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water are right and we know that this tyrant .period 1 molar therefore but the concentration because many dissociate it's a onetoone state commentary every unit of the U.S. will give you 1 unit of 8 for class and 1 unit of chloride minus started . 1 more unless you know that made associates yet again .period 1 molar of each of those now look at this and that's the basic form you know that this is the conjugated space of a week of a strong acid right where the conjugate base of a strong acid the acidic basic or neutral neutral so we know that this component is not going to affect the pH now to the other part the Cadillac look at this this is the conjugate acid of a week based Is the continent acid of a weak acidic basically neutral this is right so that means that if you want to copy the pH the component that could affect the pH would be in peaceful plus right so if for that wonderful precisely we know that if you wanna write equilibrium this it is the absence form and therefore can act as an acid in water and therefore it would be a proton donors survived taken at full blast and place it in water and we know that this is the proton donors this will be the proton accepted therefore will have each 3 0 plus the quest plus in each to request and this fall of the equation gives us 1 take all right but what they give us the Give us the key for to me to see that figure skating therefore we have to converted to figure out what he says and we know that carries 1 kW divided by TB and therefore this would be 1 times 10 to the negative for team divided by 1 . 8 times 10 negative 5 which gives me 5 .period said its times 10 to the native 10 or so that extremely weak acid so the next step is now for us to figure out what the high during my concentration is in this equilibrium and that the case was about doing that we start by saying the initial concentrations are malaria and we know that we stuck with it .period 0 1 molar of ammonium so . 0 1 will learn when not concerned about the Constitution water I just to begin with we have none of the products conform to that would be 0 then the change would be it's going to have to proceed in the forward direction because there are no products we need to find some products to establish the equilibrium so we know it's going to proceed in the forward direction there for some of the reactants can be consumed in some product is going to be formed and the state geometries 1 2 1 2 1 therefore at equilibrium I would have . 0 1 minus X X X so you can see that as a general pattern to solve all the problems of every single problem that had with it's a week after strong as the weak acid we based polyploidy gas any 1 of these examples is the same approach right so now I have to do is I don't carry a equals 5 . 6 times tend 10 which equals the high during my concentration at times the concentration of ammonia divided by the ammonium cattle are in which gives me at square of 0 . 1 0 minus X now look at the value of k values tend to the negative tended to be really really small K value so can I make an approximation here yes site case that small I know that and was small because it's going only reacted very small extent right so that means that I can make the approximation and my approximation is going to be that . 0 1 minus X is going to approximately 2 . 0 1 and words that excess that is so small that I can neglect 10 so therefore I can go back to 1965 . 6 times tend to 10 equals at Square Books .period and so is all over X X comes out to be 7 . 5 times to the negative 6 right now it would do next yes don't go ahead and quickly calculate the pH not yet you have to make sure when the assumption is correct remember that that's an assumption that we made and we have to check to see whether that assumption is appropriate OK so if I check the validity of my approximations my approximation was what that X is really small compared .period 1 so small I can neglected so that means 7 . 5 per cent in 6 smaller divided by . 1 molar times 100 and that comes out to be 7 . 5 times tend to to be assessed so that . 0 0 7 that's well below the 5 percent threshold so we do now that this approximation because and so now I can say my head during minor concentration equal status which witches 7 . 5 times tend Internet usage smaller therefore invited the pH of the that that comes out to be 5 . 1 3 to win the love of that and the pH comes out to to be 5 . 1 3 now ask itself member of presumption at the beginning was that this is the kind acid a week base therefore it has to be acidic calculated the pH comes out to be around fight when the Socialists has appeared to find Citicorp basic aren't so we know that it's in line with predictions 10 so many questions at this point so we looked at salt I'm pleased when his standing at home and during discussions on I have given you look out lots of examples of problems of this type so please make sure remember last time went to years salsa and weak acid strong acids whether you have the ability to rank them in according to relative acidic strand of basic strength in other words rank stands strong assessment the weakest pace what he reckons that the strongest base to too on the other side of the strongest assets again and we need you it's also that separate amount Academy in you as just dealing with acids alone will just look at what strong after a weak acid villainy and faces alone you have have to see whether it's a strong base for a week base right and each of you to read this and on this week's discussions were actually some examples and sold during discussion and you'll have an ample opportunity to practice that again so I wanted to could ask you that queasy something right members so let's say we have a strong acid which is a
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sea of alright an initial concentration of Seattle is 1 of the times tending to the negative to Muller 2nd in this this is a strong acid What is the key to the solution caters to write because if you don't need a calculator are right you know the pictures to right said the initial concentration of a strong acid like Seattle is tend to make it to you know that the teachers too far right now in my initial concentration of a city gas is 1 times 10 the negative 9 what would appear to the solution come to be nice now tell me it pitchers lines that solution basic control this city where strong acid another solution that is basic he know so there's a problem here and the reason is that you're in water are right up to this point in the example that we looked at concentrations of acidic gasses was so large that we know water so when you have that problem rates tend to negative 2 we know that the high during a my that comes from a others tend to the negative too but in the background you have water and how light is the concentration of hydrogen mired in pure water 10 to the negative 7 so you could get into the negative too that comes from the strong acid and attend the negative 7 that there is pure water conceded that tend to make it 7 is so small you can ignore it our right and so what we focus on is just the 10th the negative too because the difference between 10 to make it to the tendinitis Saban is enormous difference so large they can North right now when we go to a strong asset that has something like 10 to the negative not now what happens is a steel producers 10th the negative 9 new mines fuel water produces 1 10th the negative 7 to see that and so that's a big number than this so if you take a solution like the rest of the water think appeared to that solution should come up to me 7 are added to be neutral all right because now attend to the negative 7 is 100 times greater than 10 tonight at 9 all right and therefore this solution would have appeared to set right now is the difference between tendinitis Savage In the acid that is large take whichever is larger if there in approximately similar matters between something like 10 4 with it so you have an acid that sent to the use of a pure water that 87 both make some pitches approximately 7 if you have the acid that you add these extent 86 and water to the 97 solicited will difference then the problem gets more complicated because now there is the same approximate ballpark analogy used to competing equilibrium are right and so that we know we need to undergo the key to solution that is it's going to be a little more complex alright and this level of class another look at calculations where you have to competing equilibria like that but you did the moreadvanced classic analytical chemistry you will look at 2 competing equations at 2 competing equilibrium and figure out what the ultimate future that solutions all right so whenever you look at the question of this time make sure that you go back and ask yourself it might appear to consistent with where that solution is acidic obviously right and so this is examples where your acid concentrations sold to lose that now water has played important roles the came up to this point every example that we took the concentrations when acids where of the order of 1 molar .period 1 molar of . 0 1 Mullin so you healing at that concentration range you can ignore it the contribution that comes from head to undermine our right OK so long we have a common complaint looking at Chapter 11 of the text will look at acidbase histories his German want to another topic that we would like to make this equilibrium it would start looking on application of the Japanese principal carousels application of Lady Chatterley principle 2 asset base equilibrium but the end we call this the ,comma 9 effect great so it he recalled when we looked at it would be a last quarter we talk about machetes principle and we said we use lay shattered his principal To the cigarette how the system response if you disturb a system that is equivalent to the data system that's an equilibrium and disturbance somewhere we know that ultimately that system is going to go back to equilibrium and so we can make predictions about how it will respond in order go back to equilibrium became so we're looking at acidbase equilibrium soliciting example so let's say we have a weak acid so we have a teacher in water right so this is a weak acid so their fate it would be the attempt will be the proton donors want the proton accepted so can you tell me what the products of this equipment should be aides to your last stand at minus appear so this is the equilibrium so let's see we have a system a weak acid that has established this equilibrium and we know it's a weak acid because King walls 7 . 2 times 10 native the case of this axis of the gasses and select cities system has established equilibrium right after the Bolivians establish let's say we add the salt sodium chloride so all of you can recognize that we're having is assault because it's got a hand in in a cast iron and if this is a soluble salt what will happen it's and I add that salt that solution that equilibrium what is going to happen it's would associate right so this will associated in a in a blast Quest plus and minus so can only see that when I have salt to the system that could equilibrium I'm actually increasing the fluoride concentration and so by increasing the fluoride concentration and disturbing that equilibrium of power and stressing that equilibrium and Renault the correlation is principal acute disturbance equilibrium it will react in the direction in which it will counteract that stress so what was the stress of disturbance by adding extra fluoride mines so what is my stress too much fluoride so this is a response to counteract that stress so that means it has to reduce the amount of fluoride OK so we take all the only 2 directions in which you can react in 1 of 4 direction again on the reverse direction so which direction doesn't have to proceed to overcome that stresses that it use the Florida and concentration because the reverse because it goes in the forward it could produce even more Florida became so the system will respond by proceeding in the reverse direction until it goes back to equilibrium so we said yeah the system
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well responded by reactivating in reversed direction until it goes back to equilibrium right end so this shifting equilibrium position so this shift in equilibrium that the Kurds because of the addition of an iron already involved in the equilibrium of reaction is called the common currency effect because call this the commentary said so we said the shift in equilibrium position that occurs because of the addition of an iron already involved in equilibrium is called the common and effects cigarette address explain what you mean by that ,comma 9 effect it may be it describes how the system will respond you take a system that's an equilibrium and added ,comma 9 to it that's involved in that it would appear so as another example so this is an example of a weak acid soliciting example probably a week these areas ammonia survived take In addition to it was in water by now you should know that energy is a base so that would explain why can you tell me what other products even see now in is the basic study the proton accepted and what is going to be the acid therefore is the proton donors security tell what the products that reaction is going to be export last and which might get greater so this would be an 84 4 last week with and for which minus 10 and because we're right this in the form of a biggest when looking at KB and KB equals 1 point 8 times 10 to the negative fired so once again we know that this is a weak base so what we start with this and we established an equilibrium no if I have a comment on selected not to their size and assault an assault and that ammonium chloride Sinaloa and upgraded in each for us it was a plus minus sequence we know that we resolve it associates completely to produce Catalans and analyze and once again you can see that because of added :colon alliance in each 4 plus to a system that already established equilibrium 100 history that a political and money is my stress too much in each 4 plus additional classes going that means the system is going to respond represent proceeding in the reverse direction so that the reaction was shipped toward reactors and the net result there as so if you look at this and that resulted in that hydroxide and concentration decreases and therefore hydroxide iron concentration decreases what will happen to the pH of that solution pH decreases occurred it's but not in the previous example the fact is the net result is somewhat if we go back to the previous example that we looked at In the previous example if the reaction proceeds this way I write what happens if journeyman concentration because the was the equilibrium shifts towards reacted you can see that the hydrogen my concentration goes down all right so in this example the net result is that I join you mind concentration decreases and if I were a minor increase as we know pH increases right so each instance not only do you have the "quotation mark administered and you have this common an effect but is common and that actually affect the future that solution because in the 1st example when the reaction proceeds in the reverse direction you know that the high during my consultations with in the high during my contribution goes down and we know that the future and go off In a 2nd example looking at hydroxide and so in this case the net result is that proceeding this way as a hydroxide concentration decreases and therefore the pH would decrease all right so not only does this common effective disturbed that equilibrium but it also will affect the head to undermine or hydroxide and concentration so that when the company that when calculating figure out how big is this effect or a tool that can calculate what happens if you add a comment and what happens to the pH of solutions case so to do that we'll all of these problems on the class sites cell you need to copy it and show here is an example of problems involving the common line effect so the question is calculated the pH and the percentage Association of HS in a solution containing 1 more worry and . 1 molar so in Florence and you have a chance containing 1 goal in each half India anyone molar concentration of sodium fluoride cell where adding sodium fluoride so all of you recognize that because we're adding sodium fluoride we know sodium fluoride and associated to give you the Cabinet and it was started with 1 mole of sodium fluoride and we know that we end up with 1 Waller sodium Catalans and fluoride and and so the equilibrium that we're looking at is how this common I'm affects the equilibrium that we are interested in looking at which is a chair in water this is a weak acid so it can produce joint alliance plus F and we are told that Kate is 7 . 2 times 10 connected for all right then this is the fluoride ion that there so that we wanted to look at initial concentrations in malaria we know that we have 1 more of a chance to begin with we are not concerned with the concentration of water the acid hasn't reacted yet so wouldn't have 0 during a minor but of fluoride concentration is different because we added fluoride ions from that saying that the sources to begin with its Florida and concentration and effort that would be 1 so this is the only difference between the previous problems when you just look at ATF in water and we figured out the pH is now looking at is not only we have the chance to begin with a wedding assault that has at minus so before anything happens before you could was established we have 1
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more of the week after and we have 1 Moeller of the F line signs all right so now they are solved this problem is to take the same approach that we've done before so what we say is now with a look at the changes since we have a deal during informed yet we know that the reactions to proceed in the forward direction all right there for this is going to be negative acts this is the plus this would be plus that therefore at equilibrium I would have 1 minus X X this would 1 plus ET today your only difference here is that we have a minus To begin with and the concentration of the reactors preceding the board direction then that the continent F minus is going to increase all right and so that we can solve this like in equilibrium problems we can say take a equals 7 . 2 times tend to make it 4 equals the during my concentration times the Florida and concentration divided by the check and that gives me 1 plus X times x divided by 1 minus X now once again idiotic came extend to the negative for it's a small number so it's safe to make an approximation so now I can see my approximation is 1 plus X will approximate 2 1 and 1 minus X will approximate 1 as well as I was saying excess sold small compared to 1 that we can actually neglected so now I can go back to my equation I consists of 1 2 . 2 times to the native for equals 1 times divided by 1 and therefore X is 7 . 2 times tend the negative for I want check my validity is my approximation valid and I said X is really small compared to what and you can see that comes out to 7 . 2 times tend to negative 2 per cent or . 0 7 2 % all right and therefore we know that approximations pretty good it's well below 5 cent threshold it's another question was calculate the pH and Percent associations and now I calculated the pH I need hard during my concentration and calculated that the high during my concentration is X which gives me around 7 . 2 tons sent negative for therefore the pH between the negative log of that and if you work at a pH pH comes out to 3 . 1 4 right and then presented associations would be um 7 . 2 times the negative for that's S divided by the initial concentration times 100 which is 7 . 2 times tend to negative 2 per cent or as I said before . 0 7 to set fire to recalculate the key to the solution if you have a common I'm involved in that OK you know what you want to compare this to to you didn't have the common I'm in there how do you that so let's say compared to so let's compare this to compared to the absence of Indian so let's compare this to the absence of sodium fluoride survive just had a weak acid then this would be the kind of problems that we were before remember this is sort of a classic problem where what is the pH of a weak acid you given 1 molar of the acid captain with the PHS you should be able to do this kind of in sleep so I would like to write equation that is the 8th 3 or plus 8 quest plus F minus quest and I know OK 87 . 2 times 10th inning before so now I don't have any comment I'm there so my initial concentration would be just the 1 more aware of hm not consider that water since I don't have any added salt with fluoride and conservation begin the 0 therefore the change would be minus X plus 6 plus next therefore it equilibrium I would have 1 minus X X X are so if I were 2 calculation value that we can take a equals 70 . 2 times since the native for which is the high undermine concentration times the Florida's concentrations divided by the concentration of HF which gives me x squared away 1 minus X once again I can say can make that approximations I consider my approximation in this problem would be 1 minus X will approximate 2 1 2nd go back and I can say 7 . 2 times higher than the 4 equals where people over 1 and therefore and he worked that out it comes out to be 2 . 7 2 x equals 2 . 7 times 10 to the negative to the mall all right and if you check validity you can see that it's 2 . 7 times since the negative too divided by 1 more alert times 100 which gives me a 2 . 1 7 per cent to once again locate that numbers when a small can ignore it and therefore now I can say my had joined in my concentration would be 2 . 7 times tested negative to work if that were the case pH would come out to be 1 . 5 7 or write and like captured but the percentage Association its amount and associated which is 2 . 7 times tend to make it to Moeller divided by 1 molar times 100 which is 2 . 7 per cents so what do not compare this With the previous problems so in this case it I had no salt in there might appear to be 1 . 5 7 if I had to solve what is pH become 321 for city can that that illustrates that added salt would shift the quality of the reverse direction therefore the high during my concentration goes down so you can see that the hydrogen my concentration to begin with without result will be 2 . 7 times to tune with the assault will be 7 . 2 times in the next 4 all right so you can see that this has this without the solving never higher hydrogen and concentration with the salt you have a low for the day the pH the pH of the lower 320 . 5 7 you can see added insult raises the pH 1 4 you look at the Percent association without assaulted 2 . 7 % associates you have solved because 2 . 0 7 2 % so I had this all that is what is called a common on fact it affects not only the pier to the solution because we said you disturbance will remain and comment on it ,comma shift the direction in which the counteracts that stress and so the net result is that what you see here OK so
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overstocked today next classes that will be a lot of