Question

4. What molar ratio of HPO4 2-to H2PO4-in solution would produce a pH of 7.0? Phosphoric acid (H3PO4), a triprotic acid, has 3 pKa values: 2.14, 6.86, and 12.4. Hint: Only one of the pKa values is relevant here.

5. For a weak acid with a pKaof 6.0, calculate the ratio of conjugate base to acid at a pH of 5.0.

6. Which of these compounds would be the best buffer at pH 5.0: formic acid (pKa 3.8), acetic acid (pKa 4.76), or ethylamine (pKa 9.0)? Briefly justify your answer.

7. A buffer contains 0.010 mol of lactic acid (pKa 3.86) and 0.050 mol of sodium lactate per liter. (a) Calculate the pH of the buffer. (b) Calculate the change in pH when 5 mL of 0.5 M HCl is added to 1 L of the buffer. (c) What pH change would you expect if you added the same quantity of HCl to 1 L of pure water?

8. What is the pH of a solution that contains 0.20 M sodium acetate and 0.60 M acetic acid (pKa 4.76)

9. You have been observing an insect that defends itself from enemies by secreting a caustic liquid. Analysis of theliquid shows it to have a total concentration of formate plus formic acid (Ka 1.8 x 10-4) of 1.45 M; the concentration of formate ion is 0.015 M. What is the pH of the secretion?

Answer 1

4)

H₃PO₄ (aq) + H₂O (l) <------> H₂PO₄^- (aq) + H₃O⁺ (aq) Ka1

H₂PO₄^- (aq) + H₂O (l) <------> HPO₄^2- (aq) + H₃O⁺ (aq) Ka2

HPO₄^2- (aq) + H₂O (l) <------> PO₄^3- (aq) + H₃O⁺ (aq) Ka3

Ka2 contains  HPO₄^2- and H₂PO₄^-

Hence, pka2 is relevant here.

pH = pKa2 + log { [base]/[acid]}

Then,

pH = pKa2 + log {[HPO₄^2-]/[H₂PO₄^-]}

Given that pH = 7.0 , pKa2 = 6.86

Then,

7.0 = 6.86 + log {[HPO₄^2-]/[H₂PO₄^-]}

log {[HPO₄^2-]/[H₂PO₄^-]} = 0.14

[HPO₄^2-]/[H₂PO₄^-] = 10^0.14

= 1.38

Therefore,

Molar ratio of HPO₄^2- to H₂PO₄^- = 1.38