Question

A 24.3 mL sample of 0.349 M dimethylamine, (CH₃)₂NH, is titrated with 0.275 M hydrochloric acid.

(1) Before the addition of any hydrochloric acid, the pH is _____

(2) After adding 13.6 mL of hydrochloric acid, the pH is _____

(3) At the titration midpoint, the pH is _____

(4) At the equivalence point, the pH is _____

(5) After adding 45.9 mL of hydrochloric acid, the pH is _____

Equilibrium constants are found here: https://docs.google.com/document/d/1cNVOoEOVMqPSx8QdL8mc3zd8yFTwBx8VWd2MiDCDA1o/edit

Answer 1

(CH₃)₂NH , pKb = 3.27

(1) Before the addition of any hydrochloric acid, the pH is

pH = 14- 1/2 {pKb -logC}

pH = 14 - 1/2 [3.27 -log0.349]

pH = 12.14

(2) After adding 13.6 mL of hydrochloric acid, the pH is

millimoles of B = 24.3 x 0.349 = 8.48

millimoles of H+ = 13.6 x 0.275 = 3.74

B    +   H+ ----------------> BH+

8.48     3.74                      0

4.74        0                       3.74

pH = 14 - (3.27 + log (3.74 / 4.74)

pH = 10.83

(3) At the titration midpoint, the pH is

pH = 10.73

(4) At the equivalence point, the pH is

pH = 5.77

(5) After adding 45.9 mL of hydrochloric acid, the pH is

pH = 1.23