Problem 100P

If 50.0 mg of Na2CO3 are added to 150.0 mL of a solution that is 1.5 × 10−3 M in Mg2+, will any MgCO3 precipitate from the solution?

Solution 100P :

Step 1:

Molarity of tap water having Mg2+ = 1.5 × 10-3 M

Mass of Na2CO3 added = 50 mg.

Volume of solution = 150.0 mL

Will MgCO3 precipitate out of solution = ?

MgCO3 ionizes into :

MgCO3(S) Mg2+ (aq) + CO32- (aq)

We have the molarity of Mg2+. let’s calculate molarity of CO32- as :

Ksp = [Mg2+][CO32-]

Ksp of MgCO3 = 6.82 × 10-6

We are given the molarity of Mg2+, let’s substitute those values :

6.82 × 10-6 = 1.5 × 10-3 M [CO32-]

[CO32-] =

= 0.004546 M

This is the maximum soluble concentration of CO32- in the presence of 1.5 × 10-3 M Mg2+