Equilibrium · Question 1

At 25 ℃, the concentration of H+ ions in 1.00 10-3 M aqueous solution of a weak monobasic acid having acid dissociation constant (Ka) of 4.00 10-11 is X 10-7 M. The value of X is . Use: Ionic product of water (Kw) = 1.00 10-14 at 25 ℃

Answer

2.2 to 2.3

Solution

KaC = 4 10-14 is comparable to Kw, so the H+ from water cannot be neglected.

Charge balance: [H+] = [A-] + [OH-], so [H+]2 = KaC + Kw (the acid is negligibly ionised, [HA] C).

[H+]2 = 4 10-14 + 1 10-14 = 5 10-14

[H+] = 5 10-7 = 2.24 10-7 M, so X = 2.24

Answer: 2.24