Smooth Operater

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Pyruvic acid is a weak acid. Which of following is a likely pH for an aqueous solution of sodium pyruvate?

correct answer: 8.0

kaplan's Reason:
The salt of a weak acid will hydrolyze in aqueous solution according to the following generalized equilibrium expression:
A- + H2O ---> HA + -OH


but, I don't get why by looking the above reaction, I would figure out the correct pH is 8?

If you are a acid base expert, please explain. Thanks!
 

fancymylotus

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Smooth Operater said:
Pyruvic acid is a weak acid. Which of following is a likely pH for an aqueous solution of sodium pyruvate?

correct answer: 8.0

kaplan's Reason:
The salt of a weak acid will hydrolyze in aqueous solution according to the following generalized equilibrium expression:
A- + H2O ---> HA + -OH


but, I don't get why by looking the above reaction, I would figure out the correct pH is 8?

If you are a acid base expert, please explain. Thanks!

I think I remember this question. I didnt even really need to know how it dissociates because you can look at the answer choices and eliminate obviously wrong answers. Since pyruvic acid is a weak acid, the pH couldnt have been less than 7(and I think I remember that two of the answers were less than 7), and it cant be really high, like 10+(and one or two of the answers were that high). Problems like these I dont try to reason out, just use common sense&keep it moving.
 

cryptozoologist

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sodium pyruvate is the salt of the conjugate base of the weak acid. not knowing what the other choices were, 8 sounds reasonable.
 
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the acid is weak because of an unstable anion for reasons such as: charge concentration, or lower electronegativity. this means that the conjugace base of the weak acid will be a strong base. pH of a base is anything beyond neutral pH 7, but in this case, its probably less than pH 12-14 (very strong bases). so, unless the other answer choices are all within pH 8-10, then you can use process of elimination. hope that helps. also, that rxn you drew down is just a general layout of an acid base reaction provided to give you an idea of how acids and bases work, you can't derive the pH from it.
 

allstardentist

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I think you can just think of the two(pyruvic acid and its conjugate salt) as a pair of buffer solution where one is weak acid and the other one is weak base. But i dont really know why the conjugate salt is basic.
 
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