Question

Use the following information to answer the next five questions.

The following buffer was prepared:

50. mL 2.0 M methylamine (CH₃NH₂) solution and 50. mL of 2.0 M methylamine hydrochloride (CH₃NH₃Cl) solution. The K_b for methylamine is 4.4 x 10^-4.

1. What is the pH of the buffer solution?

a) 11.12                           b) 10.64                       c) 12.32                       d) 0.48                         e) 5.32

2. What is the pH of the resulting solution after adding 100 ml of 1.0 M HCl to the original buffer solution?

a) 11.12                           b) 10.64                       c) 12.32                       d) 0.48                         e) 5.32

3. What is the pH of the resulting solution after adding 200 ml of 1.0 M HCl to the original buffer solution?

a) 11.12                           b) 10.64                       c) 12.32                       d) 0.48                         e) 5.32

4. What is the pH of the resulting solution after adding 50 mL of 1.0 M KOH to the original buffer solution?

a) 11.12                           b) 10.64                       c) 12.32                       d) 0.48                         e) 5.32

5. What is the pH of the resulting solution after adding 100 mL of 1.0 M KOH to the original buffer solution?

a)11.12                           b) 10.64                       c) 12.32                       d) 0.48                         e) 5.32

Answer 1

1. What is the pH of the buffer solution?

a) 11.12                           b) 10.64                       c) 12.32                       d) 0.48                         e) 5.32

here ; K_b for methylamine is 4.4 x 10^-4, pKb = 3.36 and [BH+ ] = 2.0 M and [B]=2.0M

pOH = pKb + log10 [BH+/B]

pOH = 3.36 + log10 2.0/2.0

pOH = 3.36

pH =14-pOH

= 10.64

Hence the correct answer is 10.64.