The acid-dissociation constants of HC3H5O3 (aq) and CH3NH3+ (aq) are given in the table above. Which of the following mixtures is a buffer with a pH of approximately 10.5?

Question

Given:

HC3H5O3--> Ka: 8.3*10^-4

CH3NH3+---> Ka: 2.3*10^-11

The acid-dissociation constants of HC₃H₅O₃ (aq) and CH₃NH₃⁺ (aq) are given in the table above. Which of the following mixtures is a buffer with a pH of approximately 10.5?

1. A mixture of 100. mL of 0.1 M CH₃NH₃Cl and 50. mL of 0.1 M NaOH

1. A mixture of 100. mL of 0.1 M HC₃H₅O₃ and 50. mL of 0.1 M NaOH

1. A mixture of 100. mL of 0.1 M HC₃H₅O₃ and 100. mL of 0.1 M NaC₃H₅O₃

1. A mixture of 100. mL of 0.1 M NaC3H5O3 and 100. mL of 0.1 M NaOH

(I think it is the 3rd one not sure though)

Answer 1

Not sure why you chose the 3rd one which is a mixture of a weak ACID and the conjugate base. Since the acid has a Ka of 8.3x10-4, the pKa is ~ 3.1. This would be a good choice for a buffer with a pH around 3-4. To make a buffer with pH 10.5, you want something with a pKa close to the desired pH of 10 - 11. The CH3NH3+ has a pKa = 10.6, so it would be a good choice.

The only option given that contains this acid is the first choice of CH3NH3Cl + NaOH. This is the same as

CH3NH3+ + OH- ==> CH3NH2 + H2O. The buffer would thus be the weak acid CH3NH3+ and the conjugate base, CH3NH2. Since the volume of 0.1 M NaOH is less than the volume of 0.1 M CH3NH3Cl, the buffer would exist.