Precipitation: Q vs K, Calculate

Question

When 12.0 mL of a 1.73×10^-4 M sodium hydroxide solution is combined with 15.0 mL of a 3.79×10^-4 M cobalt(II) bromide solution does a precipitate form? (yes or no)

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

Answer 1

If Q > Ksp, a precipitate will form, otherwise it will not.

2NaOH(aq) + CoBr₂(aq) ==> 2NaBr(aq) + Co(OH)2(s)

Co(OH)₂(s) <==> Co²⁺(aq) + 2OH^-(aq) ... Ksp = ?? look it up.

Q = [Co²⁺][OH-]²

[Co²⁺] = 15.0 ml x 1 L/1000 ml x 3.79x10^-4 mol/L = 5.7x10^-6 mol/0.027 L = 2.1x10^-4 M Co²⁺

[OH-] = 12.0 ml x 1 L/1000 ml x 1.73x10^-4 mol/L = 2.1x10^-6 mol/0.027 L = 7.7x10^-5 M OH-

Q = (2.1x10^-4)(7.7x10^-5)² = 1.25x10^-12

Compare this value to Ksp and determine if a ppt forms.