Question

Find the pH of each of the following solutions of mixtures of acids. 4.5×10⁻² M in acetic acid and 4.5×10⁻² M in hydrocyanic acid

Answer 1

The Ka value of hydrocyanic acid(HCN) very small in comparison to the Ka value of acetic acid(CH3COOH). Hence the H+ ion produced by HCN is very small in comparison to the H+ ion produced by CH3COOH and hence can be neglected.

The chemical reaction for the dissociation of CH3COOH is

------------------- CH3COOH ----- > CH3COO- + H+ ; Ka = 1.74*10^-5

Initial.con(M): 0.045, ---------------- 0 --------------- 0

--Change(M): - 0.045x, --------------- +0.045x, + 0.045x

eqm.conc(M):0.045(1 - x), ---------- 0.045x, ---- 0.045x

Ka = 1.74*10^-5 = (0.045x)*(0.045x) / 0.045(1 - x)

Since x << 1, (1-x) is nearly equals to 1.

=> x = 0.019664

Hence [H+] = 0.045x = 0.045 * 0.019664 = 8.849x10^-4 M

=> pH = - log[H+] = - log(8.849x10^-4 M) = 3.05 (answer)