Precipitation: Q vs K, Calculate

Question

When 25.0 mL of a 6.93×10^-4 M silver acetate solution is combined with 18.0 mL of a 7.24×10^-4 M ammonium sulfide solution does a precipitate form? fill in the blank 1 _______(yes or no)

For these conditions the Reaction Quotient, Q, is equal to _____.

Answer 1

To determine if a precipitate forms, we need to compare the Q to the Ksp for silver sulfide (Ag₂S). You don't provide the Ksp for Ag2S, and I'm too lazy to look it up, but I'll show you how to do the problem, and then you can look up the value.

2AgC₂H₃O₂(aq)+ (NH₄)₂S(aq) ==> 2NH₄C₂H₃O₂(aq) + Ag₂S(s)

moles Ag⁺ = 0.025 L x 6.93x10^-4 mol/L = 1.7x10^-5 moles Ag⁺. [Ag⁺] = x1.7x10^-5 moles/0.043L = 3.95x10^-4M

moles S^2- = 0.0180 L x 7.24x10^-4 mol/L = 1.3x10^-5 moles S^2-. [S^2-] = 1.3x10^-5 moles/0.043L = 3.0x10^-4M

Ag₂S(s) <==> 2Ag⁺(aq) + S^2-(aq) and Q = [Ag⁺]²[S^2-]

Q = (3.95x10^-4)²(3.0x10^-4)

Q = 4.7x10^-11

Compare this value to the Ksp for Ag2S.

If Q >Ksp, then a precipitate will form

If Q<Ksp, then no precipitate will form