Question

You made up a saturated solution of calcium sulfate (CaSO4). The temperature is 25°C. You then add 5.00 × 10−3 M sodium sulfate (Na2 SO4 ). What are the concentrations of calcium and sulfate after equilib- rium is reached? The pKs of CaSO4 is 4.5

Answer 1

Answer:

[Ca²⁺] = 0,00365M

[SO₄²⁻] = 0,00865M

Explanation:

The reaction:

CaSO₄(s) ⇄ Ca²⁺ + SO₄²⁻ has a Ksp of 10^-4.5 = 3,16x10⁻⁵ = [Ca²⁺]  [SO₄²⁻] (1)

Concentrations are

[Ca²⁺] = X

[SO₄²⁻] = X

The addition of 5.00x10⁻³M of ions of SO₄⁻ gives a concentration of:

[Ca²⁺] = X

[SO₄²⁻] = (X + 0,005M)

Replacing in (1):

3,16x10⁻⁵ = X (X + 0,005M)

3,16x10⁻⁵ = X² + 0,005X

0 = X² + 0,005X - 3,16x10⁻⁵

Solving for X:

X = -0,00865M

X = 0,00365M

Thus, concentrations in equilibrium are:

[Ca²⁺] = 0,00365M

[SO₄²⁻] = 0,00365M + 0,005M = 0,00865M

I hope it helps!