water that contains ca2+

Question

Water containing Ca2+ and Mg2+ ions is called hard water and is unsuitable for some household and industrial use because these ions react with soap to form insoluble salts. One way to remove the Ca2+ ions from hard water is by adding washing soda (Na2CO3). The molar solubility of CaCO3 is 9.3×10-5 M. What is its molar solubility in a 0.050 M Na2CO3 solution?

Answer 1

This is a "common ion" problem.

Let us first calculate the Ksp for CaCO3 from the given molar solubility:

CaCO₃(s) <===> Ca²⁺(aq) + CO₃^2-(aq)

Ksp = (9.3x10^-5)² = 8.65x10^-9

 

The common ion is CO₃^2- at 0.050 M (neglect that provided by CaCO₃ as it will be very small relative to 0.05 M)

8.65x10^-9 = (x)(0.05)

x = 1.73x10^-7 M = molar solubility in the presence of 0.05 M Na₂CO₃