You mix equal volumes of 0.05 M Na 2 HPO 4 and 0.05 M Na 2 HPO 4 (pk\'s for phos

Question

You mix equal volumes of 0.05 M Na2HPO4 and 0.05 M Na2HPO4 (pk's for phosphoric acid are 2.15, 6.82 and 12.38). Which of the following best describes the resulting solution?

A) pH 2.15 and poorly buffered

B) pH 2.15 and well buffered

C) pH 6.82 and well buffered

D) pH 12.38 and well buffered

E) pH 6.82 and poorly buffered

* From using the Henderson Hasselbach equation, I can see that 0.05 M/ 0.05 M = 0, so my pH is dependent on the pKa. I just can't figgure out why I would choose the pKa of 6.82 and why it's supposed to be well buffered instead of poorly buffered.

Explanation / Answer

The salt given in the question is Na2HPO4 which is the salt of biphosphate (HPO4^(-2)) which is obtained from H3PO4 as follows

H3PO4= H2PO4^(-) + H^(+), H2PO4^ (-)=HPO4^(2-) + H^+. Hence,the second pka value of H3PO4 i.e. 6.82 is to be considered. It's well buffered since the H3PO4 is a weak acid and Na2HPO4 is a salt of weak acid thus fulfilling the criteria of ideal buffer solution.

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