What is the pH with the activities?

Question

Determine the pH of the same solution including activities. Activity coefficients at various ionic strengths can be found in this table.

solution contains 0.045M NaOH and 0.025M KI

I was able to calculate the pH by neglecting the activities (first part of the question) but cannot figure it out with the activities.

Answer 1

0.045 M NaOH ==> 0.045 M Na⁺ + 0.045 M OH^-

0.025 M KI ==> 0.025 M K⁺ + 0.025 M I^-

Ionic strength (u) = 1/2 ∑CZ² = 1/2 (0.045 + 0.045 + 0.025 + 0.025) = 0.07

The closest u in the table is 0.05, unless you were taught how to extrapolate

Activity coefficient (γ) for OH- @ u = 0.05 = 0.81

pOH = -log{OH-} = -log [OH-]γ = -log (0.045)(0.81) = log 0.03645

pOH = 1.438

pH = 12.56

Note: there are various methods of calculating u and γ, so if you have another way to calculate u, use it. The current method ignores ionic size, for example.