Question

Sodium phosphate is added to a solution that contains 0.0077 M aluminum nitrate and 0.031 M calcium chloride. The concentration of the first ion to precipitate (either Al3+ or Ca2+) decreases as its precipitate forms. What is the concentration of this ion when the second ion begins to precipitate?

Answer 1

For Ca₃(PO₄)₂, the Ksp value is 2*10^-29.

For AlPO₄, the Ksp value is 6*10^-19.

Aluminium phosphate will precipitate first because it has lesser value of solubility in terms of moles per liter.

When the second ion, i.e., Ca²⁺, starts precipitating, we have:

[Ca]³*[PO4]² = [0.031]³*[PO4]² = 2*10^-29

Solving we get:

[PO₄] = 8.19*10^-13 M

When phosphate conc is this, then the conc of Al³⁺ ion will be:

[Al]*[PO4] = [Al]*[8.19*10^-13] = 6*10^-19.

Solving we get:

[Al³⁺] = 7.33*10^-7 M

Hope this helps !