The solubility of Ag2CrO4 in water is 20.9 mg in 1.0 L of water at 25 °C. What is the value of Ksp for Ag2CrO4?

Molar mass of Ag₂CrO₄= (107.9×2 + 52.0 + 16.0×4) g/mol = 331.8 g/mol
Solubility of Ag₂CrO₄ = [(20.9/1000 g) / (331.8 g/mol)] / (1.0 L) = 6.3 × 10⁻⁵ M

Ag₂CrO₄(s)   ⇌   2Ag⁺(aq)        +        CrO₄²⁻(aq)         Ksp
Initial:                    0 M                           0 M
Change:      (2 × 6.3 × 10⁻⁵ M)       +(6.3 × 10⁻⁵ M)
Eqm:    　　 2 × 6.3 × 10⁻⁵ M            6.3 × 10⁻⁵ M

At equilibrium:
Ksp
= [Ag⁺]² [CrO₄²⁻]
= (2 × 6.3 × 10⁻⁵)² (6.3 × 10⁻⁵)
= 1.0 × 10⁻¹²