H3PO4 (aq) + H2O (l) ⇔ H2PO4- (aq) + H3O+ (aq) Ka = 7.1 x 10-3 A buffer was prepared using 0.00650 M H3PO4 and 0.0125 M NaH2PO4. Calculate the pH of the buffer solution

Question

H₃PO₄ (aq) + H₂O (l) ⇔ H₂PO₄^- (aq) + H₃O⁺ (aq)      K_a = 7.1 x 10^-3

A buffer was prepared using 0.00650 M H₃PO₄ and 0.0125 M NaH₂PO₄. Calculate the pH of the buffer solution

Answer 1

Hi Anya, Please see the video above. While the other answer given would normally be correct, in this case there is a twist. Also, it is very important that you go away with not just the answer, but with the general knowledge of how to solve this problem. I can guarantee that you will see another version, possibly more difficult, of this on any tests you need to pass for your course. - Greg.

Answer 2

pH = pKa + log [conjugate base]/[acid] ... Henderson Hasselbalch equation

pKa = -log Ka = -log 7.1x10-3 = 2.15

pH = 2.15 + log [0.0125/0.00650) = 2.15 + log 1.92

pH = 2.15 + 0.28

pH = 2.43