Question

Consider a solution of 2.0 M HCN and 1.0 M NaCN (Ka for HCN = 6.2 x 10-10). Which of the following statements is true?

a) The solution is not a buffer because [HCN] is not equal to [CN-]

b) The pH will be below 7.00 because the concentration of the acid is greater than that of the

base.

c) [OH-] > [H+]

d) The buffer will be more resistant to pH changes from addition of strong acid than to pH changes from addition of strong base.

e) All of these statements are false.

Which of the following solutions will be the best buffer at a pH of 9.267? (Ka for HC2H3O2 is 1.8 x 10-3; Kb for NH3 is 1.8 x 10-5)

a. 0.20 M HC2H3O2 and 0.20 M NaC2H3O2

b. 3.0 M HC2H3O2 and 3.0 M NH4Cl c. 0.20 M NH3 and 0.20 M NH4Cl
d. 3.0 M NH2 and 3.0 M NH4Cl
e. 3.0 M HC2H3O2 and 3.0 M NH3

Answer 1

The solution is not a buffer because [HCN] is not equal to [CN–].
A buffer is composed of a weak acid and its salt which are both present in the solution.

All of the other statement choices are false.
Since the next statement is true then this statement is false.

[OH–] > [H+]
Since pH = 8.91, see below, the statement is correct.

The buffer will be more resistant to pH changes from addition of strong acid than to pH changes from addition of strong base.
Since the [CN^–] is less than the [HCN] addition of acid will just use up the CN^- and the pH will decrease faster toward lower pH.

The pH will be below 7.00 because the concentration of the acid is greater than that of the base.
pH = pKa + log[CN^-]/[HCN] = 8.91. The term 'below' usually means more acidic, therefore, the statement is false.