Calculate the solubility of MCl in the water taking activity effects into account. As MCl dissolves, the ionic strength increases so there is a feedback effect: when MCl dissolves, it increases μ, which decreases activity coefficients, which allows more MCl to dissolve, which increases μ, which... You will have to solve this recursively. Repeat the cycle of calculations until you get two successive answers that agree to two significant figures. Use the value α = 250 pm for the M⁺ ion.

Ksp= 5.9E-3