Working out Ks from solubility

• What is the Ks for CaCO3(s) when s(CaCO3(s))= 5.01 x 10-6 mol L-1 ?

Write the equation for the solid at equilibrium with water

CaCO3(s) Ca2+ + CO32-

Work out the concentrations of the aqueous ions.

If the solubility of CaCO3(s)= 5.01 x 10-6 mol L-1 [Ca2+] = 5.01 x 10-6 mol L-1 [CO3

2-] = 5.01 x 10-6 mol L-1

Write the Ks expression. Remember do not include water or solids into the expression.

Ks = [Ca2+] [CO32-]

Substitute the numerical values for [Ca2+] and [CO3

2-] into the expression.

Ks = [5.01 x 10-6] [5.01 x 10-6]

Ks = [2.51 x 10-11]

• What is the Ks for Ag2S(s) 1.26 x 10-17 moles of Ag2S(s) dissolves in 1 litre of water?

Write the equation for the solid at equilibrium with water

Ag2S(s) 2Ag+ + S2-

Work out the concentrations of the aqueous ions.

If the solubility of Ag2S(s)= 1.26 x 10-17 mol L-1 [Ag+] = 2 x s(Ag2S) = 1.26 x 10-17 mol L-1

[S2-] = s(Ag2S) = 2.52 x 10-17 mol L-1

Write the Ks expression. Remember do not include water or solids into the expression.

Ks = [Ag+]2 [S2-]

Substitute the numerical values for [Ag+] and [S2-] into the expression.

Ks = [2.52 x 10-17]2 [1.26 x 10-17]

Ks = [8.00 x 10-51]