Ch.17 Practice Problems CHM 132
1. (a) Calculate the pH of a buffer that is 0.12 M in lactic acid and 0.11 M in sodium lactate. (b) Calculate the pH of a buffer formed by mixing 85mL of 0.13 M lactic acid with 95 mL of 0.15 M sodium lactate. (Ka for lactic acid = 1.4 x 10-4) Ans. a) 3.82 b) 3.96
2. (a) A buffer solution contains 0.10 mol of acetic acid and 0.13 mol of sodium acetate in 1.00 L. (a) What is the pH of this buffer (b) What is the pH of the buffer after the addition of 0.02 mol of KOH? (c) What is the pH of the buffer after the addition of 0.02 mol of HNO3? (Ka for acetic acid = 1.8 x 10-5)
Ans. a) 4.86 b) 5.02 c) 4.71
3. (a) The figure to the right shows the titration curves for two monoprotic acids. (a) Which curve is that of a strong acid? (b) What is the approximate pH at the equivalence point of each titration? (c) How do the concentrations of the two acids compare if 40.0mL of each is titrated with the same concentration of base?
Ans.
a) B
b) B = 7 and A = 8
c) For equal volumes of A and B, the concentration of acid B is greater since it requires a larger volume of base to reach the equivalence point.
4. Two monoprotic acids, both 0.100 M in concentration, are titrated with 0.100 M NaOH. The pH at the equivalence point for HX is 8.8 and that for HY is 7.9.Which is the weaker acid?
Ans. HX. At the equivalence point the X- ions acts like a conjugate base. The weaker the acid, the stronger the conjugate base, and the more basic the solution will be at the equivalence point.
5. A 35.0 mL sample of 0.150 M acetic acid, HC2H3O2 is titrated with 0.150 M NaOH solution. Calculate the pH after the following volumes of base have been added:
(a) 0 mL (b) 17.5 mL (c) 34.5 (d) 35.0 mL (e) 35.5 mL (f) 50.0 mL
Ans. a) 2.78 b) 4.74 c) 6.58 d) 8.81 e) 11.03 f) 12.42