Question 3 of 18 incorrect incomect Incorrect University Science Book Donald McQ

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Question 3 of 18 incorrect incomect Incorrect University Science Book Donald McQuarrie Peter A. Rock.Ethan Gallogly presented by Sapling Lean Phosphoric acid is a triprotic acid (Ka1 6.9x 10 3, Ka2 6.2x10 8, and Ka3 s 4.8x10 To find the pH of a buffer composed of H2PO4 (aq) and HPO42 (aq), which pKa value would you use in the Henderson-Hasselbalch equation? Incorrect. Phosphoric acid ionizes in three steps: O pKa1 2.16 H,PO (ag) O pKa2 7.21 Which equation contains both of the H (aq) HPOz (ag) in the buffer? H,PO O pKa3 12.32 H (aq) +PO HIPO the pHof a obtained by dissoiving Tao gorRHPoas and 32ogof NaaHPods in water and then diluting to 1.00 L. Number pH Next Exit

Explanation / Answer

Hi

A) pKa2= 7.21 is the answer

B) For a buffer use the Henderson-Hasselbalch equation

the relevant ions are HPO4(^2-) and H2PO4(^-); The latter is the acid, and the former is its conjugate base. The equilibrium constant is Ka2 for phosphoric acid (dissociation of the second H+). The Na+ and K+ ions are spectator ions and do not participate.

pH = pKa2 + log{[HPO4(^2-)]/[H2PO4(^-)]}
Ka2 = 6.2 X 10^-8
pKa2 = 7.21
[HPO4(^2-)] = 32/ 141.96= 0.2254 M
[H2PO4(^-)] = 14/136.086= 0.1029 M

pH = 7.21 + log[0.2254/0.1029] = 7.55

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