Question

Calculate the p H and the concentration of all species present (H₃  O⁺ , A - , HA, and OH - in 0.50 M HA. K_a = 3.5 x 10^-6

Answer 1

Weak acid dissociates as AH + H₂O ⇌ A- + H₃O+

K_a = [A^-][H₃O⁺] / [AH]

but  [A^-] = [H₃O⁺] = x

K_a = [x][x] / [AH]

Substitute the value in equation

3.510^-6 = [x]²/ 0.50

[x]² = 3.510^-60.50 = 1.7510^-6

[x] = 1.322910^-3 M

Concentration of H₃O⁺ = 1.322910^-3 M

pH = - log[H₃O⁺]

pH = - log 1.322910^-3

pH = 2.88

pOH = 14 - pH = 14 - 2.88 = 11.12

[OH^-] = 10^-pOH = 10^-11.12 = 1.586 10^-12

[OH^-] = 1.586 10^-12

concentration of HA after dissociation = (initial concentration of HA) - [H₃O⁺]

= (0.50) - (1.322910^-3) = 0.4986771 M

[HA] = 0.4989771 M