Question

Lead(II) chromate, PbCrO4, was used as a yellow paint pigment (“chrome yellow”). When a solution is prepared that is 5.0 × 10–4 M in lead ion, Pb2+, and 5.0 × 10–5 M in chromate ion, CrO42–, would you expect some of the lead(II) chromate to precipitate? The solubility product constant for lead(II) chromate is 2 × 10–16.

Answer 1

Q_ip is the ion product reaction quotient and is based on initial conditions of the reaction. The solubility quotient (often called the ion product) has the same form as the solubility constant expression, but the concentrations of the substances are not necessarily equilibrium values. Rather, they are the concentrations at the start of the reaction.

To predict if a precipitation occurs:

• Precipitation should occur if Q_ip > Ksp (reaction should go in reverse direction and precipitation should occur)

• Precipitation cannot occur if Q_ip < Ksp (the solution is unsaturated)

• A solution is just saturated if Q_ip = Ksp (the reaction mixture is at equilibrium)

Now giveb the, Concentration of pb²⁺ = 5.0× 10^–4 M in lead ion,

Concentration of CrO₄^2- = 5.0 × 10^–5 M in chromate ion,

Q_ip = [pb²⁺]_initial [CrO₄^2-]_initial

Q_ip = [5.0× 10^–4 M] [5.0× 10^–4 M]

Q_ip = 2.5 × 10^-9

Ksp is given as =2 × 10^–16.

since Q_ip > Ksp we can say that precipitation of lead(II) chromate will occur.