Question

The molar solubility of calcium phosphate in a 0.288 M potassium phosphate solution is ..?

Answer 1

Answer:

5.116x10⁻⁹ M

Explanation:

First, let's know the molecular formula of calcium phosphate. Calcium is a metal of group 2, so it loses 2 electrons in an ionic bond and forms the cation Ca⁺². Phosphate is the ion PO₄⁻³, so to form the compound, first comes the cation, then the anion and the charges must change between then:

Ca₃(PO₄)₂

The chemical equation for its dillution is:

Ca₃(PO₄)₂(s) ⇄ 3Ca⁺² (aq) + 2PO₄⁻³(aq)

In the solution, there is ions phosphate from potassium phosphate. Potassium is from group 1, so it loses 1 electron and forms the ion K⁺, so: K₃PO₄. It means that in 1 mol of the salt, will be 1 mol of phosphate. Then, the initial concentration of phosphate is 0.288M. So:

Ca₃(PO₄)₂(s) ⇄ 3Ca⁺² (aq) + 2PO₄⁻³(aq)

M                         0                 0.288               Initial

-S                       +3S                +2S                 Reacted

M-x                      3S           0.288 +2S           Equilibrium

S is the molar solubility. In the expression of the equilibrium constant (Kps) the solids don't participate. And the concentration must be raised to the coefficient, so:

Kps = [Ca⁺²]³x[PO₄⁻³]²

1.0x10⁻²⁵ = (3S)³x(0.288 + 2S)²

1.0x10⁻²⁵ = (9S³)x(0.083 + 1.152S + 4S²)

1.0x10⁻²⁵ = 0.747S³ + 10.368S⁴ + 36S⁵

Which is an equation of the 5th grade, the equation must be solved by try and error or by a computer.

S = 5.116x10⁻⁹ M

Answer 2

Answer:

3.55 × 10⁻⁹ M

Explanation:

Potassium phosphate is a strong electrolyte that dissociates according to the following equation.

K₃PO₄(aq) → 3 K⁺(aq) + PO₄⁻(aq)

If the concentration of K₃PO₄ is 0.288 M, the concentration of PO₄⁻ will be 0.288 M.

We can find the molar solubility (S) of calcium phosphate using an ICE chart.

          Ca₃(PO₄)₂ ⇄ 3 Ca²⁺ + 2 PO₄³⁻

I                                    0         0.288

C                                +3S         +2S

E                                  3S      0.288 + 2S

The solubility product (Ksp) is

Ksp = 1.0 × 10⁻²⁵ = [Ca²⁺]³.[PO₄³⁻]²= (3S)³(0.288 + 2S)²

Since 2S << 0.288, we can neglect it to simplify the calculations.

Ksp = 1.0 × 10⁻²⁵ = (3S)³(0.288)²

S = 3.55 × 10⁻⁹ M