Problem summary
pKa(CH3NH3+) = 10.64
Calculate the pH of 0.10 CH3NH2.
To calculate the pH of the solution:
Relate pKa (Ka) and pH (H3O+) using
CH3NH2 + H2O Relate pKa (Ka) and pH (H3O+) using
the Ka expression
the reaction to reach equilibrium and Kw
| Ka = | [CH3NH2][H3O+] |
| [CH3NH3+] |
CH3NH3+ + OH–Kw = [H3O+] [OH–]
Relate [CH3NH2] to initial concentration.
[CH3NH2] ≈ 0.10
Relate [CH3NH3+] to pH (H3O+) using the reaction to reach equilibrium and Kw
[CH3NH3+] = [OH–]
[CH3NH3+] = Kw/[H3O+]
[CH3NH2] ≈ 0.10
Relate [CH3NH3+] to pH (H3O+) using the reaction to reach equilibrium and Kw
[CH3NH3+] = [OH–]
[CH3NH3+] = Kw/[H3O+]
| 2.29 × 10–11 = | 0.10 × [H3O+] |
| 10–14 /[H3O+] |
pH = 11.82
Entering the above into the Ka expression:
[OH–] = 6.61 × 10–3 (calculated using Kw).
Thus the approximation that the extent of reaction to reach equilibrium is low is valid. Note that the pH of a solution containing only the base of a weak acid-base conjugate pair as major species has a pH higher than pKa for the conjugate pair.
Thus the approximation that the extent of reaction to reach equilibrium is low is valid. Note that the pH of a solution containing only the base of a weak acid-base conjugate pair as major species has a pH higher than pKa for the conjugate pair.
Note that the pH is higher than pKa for the conjugate acid.
The pH of an aqueous solution prepared by dissolving only the base of a weak acid-base conjugate pair is BOTH higher than 7 and higher than pKa for the conjugate pair.