Answer to Question #141247 in General Chemistry for Christine

Question

Will a precipitate form if 120 mL of 0.045 M silver nitrate is mixed with 180 mL of 0.052 M sodium
chromate? (silver chromate has Ksp = 1.9 x10^–12) Calculate [Ag+] and [CrO4^2–] at equilibrium.

Answer 1

2Ag⁺ (aq) + CrO₄^2- (aq) ↔ Ag₂CrO₄ (s)

mol Ag⁺ (aq) = (0.120 L)(0.045 mol/L) = 0.0054 mol Ag⁺

[Ag⁺] = 0.0054 mol/(0.120 L + 0.180 L) = 0.0054 mol / 0.300 L = 0.018 M

mol CrO₄^2- (aq) = (0.180 L)(0.052 mol/L) = 0.00936 mol CrO₄^2-

[CrO₄^2-] = 0.00936 mol / 0.300 L = 0.0312 M

Q = [Ag⁺]²[CrO₄^2-] = (0.018)²(0.0312) = 1.01 x 10^-5

Q > K_sp

Ag₂CrO₄ will precipitate from solution.

At the point where Q = K_sp the solution is at equilibrium.

Assumption: [Ag⁺] = [CrO₄^2-]

1.9 x 10^-12 = [Ag⁺]²[Ag⁺] = [Ag⁺]³

[Ag⁺] = [CrO₄^2-] = 1.24 x 10^-4 M (at equilibrium)