Change in pH

[phaloz]

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Which of the following will result in no change in pH?

- adding NH4NO3 to NH3solution
- adding KCl to HCl solution
- adding HCOONa to a HCOOH solution

I am thinking KCl to HCl because Cl- doesn't have any affinity for H+ ions.

 

[SaintJude]

Isn't it the last option?

Edit: It depends what you are asking. The last choice is a buffer solution, so it (a mixture containing both) will maintain its pH.

But if you're asking if there is HCl in a beaker and then you add KCl, then I would say option 2 as well.

 

[typicalindian]

Yeah isn't the last one a conjugate acid base pair?

Edit: I think your right. It might be the second one because KCl is a neutral salt so it wont change pH

 

[phaloz]

Thanks guys, I really needed some confirmation on that one.

Boy these acid base titration problems are incessantly annoying 👎; not this problem in particular, but the ones where you have to find out the pH of the buffer and what not.

 

[MedPR]

Thanks guys, I really needed some confirmation on that one.

Boy these acid base titration problems are incessantly annoying 👎; not this problem in particular, but the ones where you have to find out the pH of the buffer and what not.

Just use HH equation. It's probably going to be one of the plug and chug problems on the MCAT.

 

[indianjatt]

I believe the first one is a good buffer too. It is a classic example of a buffer. We have nh4+ which is the conjugate acid and nh3 is the conjugate base.

 

[indianjatt]

But at the same time for HCl, doesn't the fact that were adding a common ion shift the equilibrium to the left.. Making the solution more basic and thus indicating a change in ph? Whereas for buffets, we usually hover around the same pH if we have a relatively equal concentration of conjugate acid/conjugate base.

 

[MedPR]

Which of the following will result in no change in pH?

- adding NH4NO3 to NH3solution
- adding KCl to HCl solution
- adding HCOONa to a HCOOH solution

I am thinking KCl to HCl because Cl- doesn't have any affinity for H+ ions.

What's the answer? The question is a little vague.

 

[chiddler]

All of them, yes?

First and last are buffers, middle is like adding table salt to an acid solution.

 

[MedPR]

All of them, yes?

First and last are buffers, middle is like adding table salt to an acid solution.

Wouldn't the pH change for 1 and 3 be dependent on how much you are adding? If you do weak base/weak acid buffer, don't they have to be within 0.1M concentrations of each other to make a buffer?

 

[chiddler]

Wouldn't the pH change for 1 and 3 be dependent on how much you are adding? If you do weak base/weak acid buffer, don't they have to be within 0.1M concentrations of each other to make a buffer?

Yes but, as an mcat question, i'll be willing to bet that it works out as a buffer system and within threshold. Also because he didn't give more information.

Not necessarily within 0.1 M concentrations. You can just add more of a dilute concentration or less of a concentrated one.

 

[MedPR]

Yes but, as an mcat question, i'll be willing to bet that it works out as a buffer system and within threshold. Also because he didn't give more information.

Not necessarily within 0.1 M concentrations. You can just add more of a dilute concentration or less of a concentrated one.

Oh yea, the rule I'm thinking of is pKa +/- 1.

 

[pfaction]

Wait, isn't it B? K+ and CL- won't change any pH. NH4NO3 would be acidic (NH4 is weakly acidic and NO3 is a conjugate of HNO3, so nothing)....

 

[milski]

Yes, it's B. Making buffers will change the pH of the weak acid when the conjugate is added.

 

[SaintJude]

Glad you posted this question: Did not notice that NH4NO3 is a weak acid, but totally makes since since it's a product of a strong acid & weak base.

 

[pfaction]

Yes, it's B. Making buffers will change the pH of the weak acid when the conjugate is added.

I was about to ask this as well. TPR says any time a base is added, no matter if it's a buffer or not, the pH will increase.

- adding NH4NO3 to NH3solution

In this case, NH4+ and NO3-. NO3- is conjbase of HNO3=no activity, NH4=weak acid. Acidic solution.

- adding KCl to HCl solution
K+ is group1=no, CL-strongacidconj=no = neutral, like adding NaCl to H2O

- adding HCOONa to a HCOOH solution
Adding base to acid = solution becomes basic

 

[Samchik]

But at the same time for HCl, doesn't the fact that were adding a common ion shift the equilibrium to the left.. Making the solution more basic and thus indicating a change in ph? Whereas for buffets, we usually hover around the same pH if we have a relatively equal concentration of conjugate acid/conjugate base.

As far as I remember, common-ion stuff is only relevant with partially soluble compounds. However, HCl and KCl are completely (~100%) soluble, thus common-ion effect is not applicable here.

The correct answer is B. KCl is a neutral salt and will not change pH.

 

[Morsetlis]

Buffers absolutely change the pH. They are called buffers because they MINIMIZE changes in pH.

 

[phaloz]

As far as I remember, common-ion stuff is only relevant with partially soluble compounds. However, HCl and KCl are completely (~100%) soluble, thus common-ion effect is not applicable here.

The correct answer is B. KCl is a neutral salt and will not change pH.

Yes, this is indeed why. The reaction (HCl ----> H+ + Cl-) is not an equilibrium reaction, because the HCl dissociates 100%. The Cl- as no affinity to react with H+, hence there is no reverse reaction and the pH (the H+ concentration) remains unchanged.