Amphiprotic Substance x

[silverice]

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Substance x is amphiprotic pka=5.7 which of the following is true about the ph of a 3 M solution of substances x
A. Ph is less than 7
B. ph is 7
C. Ph is more than 7
D. Need more information to determine the ph

 

[ljc]

Substance x is amphiprotic pka=5.7 which of the following is true about the ph of a 3 M solution of substances x
A. Ph is less than 7
B. ph is 7
C. Ph is more than 7
D. Need more information to determine the ph

My initial guess is that the Ph should be less than 7. This is because when you put a substance with some pKa is a solution, it will donate protons when pH of solution > pKa, or accept them if pH of solution < pKa. So this substance wants to move towards pH of 5.7.

So I'd guess A. What is the answer?

Edit: This can (probably) be solved using Henderson-Hasselbalch, but I'm really rusty on that so I'll refrain. Someone else can jump in if they like.

 

[chiddler]

could the description include it being in the form of conjugate base salt? that would make it D.

no that doesn't make sense. for it to be amphiprotic it must contain acid and base. So it must be pH less than 7 because there is an acidic proton.

It's A, right?

 

[silverice]

I was leaning towards A, but the answer is D.

Explanation: although we know the pka of this substance, we do not know its pkb. we cannot simply subtract its pka from 14, since doing so would give the pkb of the substance's conjugate. therefore, since w don't know whether this substance is a more effective acid or base, we cannot determine what the ph will be.