Question

What is the maximum concentration og mg2+ that can exist in solution at pH 12?

How much hardness does this represent, expressed as mg/L CaCO3? (hint: Kso of Mg(OH)2 = 10^-11.8)

THIS IS KSO, NOT KSP. PLEASE DONT USE KSP.

Answer 1

Given that; pH =12

pH + pOH = 14

pOH = 14-12= 2

[OH-] =10^-pOH

[OH-] =10^-2

[OH-] = 0.010 M

Given that solubility product Kso of Mg(OH)2 = 1.8 * 10^-11.

1.8 x 10^-11 = [Mg2+] ( 0.010)^2

[Mg2+] = 1.8 x 10^-7 M or mole / L

Calculating Magnesium Hardness as CaCO3

To calculate magnesium hardness, we use this Equation

magnesium hardness (m/L) as CaCO3/equivalent weight of CaCO3

= magnesium (mg/L)/equivalent weight of magnesium

[Mg2+] = 1.8 x 10^-7 M or mole / L

= 24.305 g/mol *1.8 x 10^-7 mole / L

= 4.3749*10^-6 g/ L

= 0.0044 mg/ L Mg2+

x mg/L /50.045=0.0044 mg/L /12.15

x=0.0044 x 50.045/12.15

=0.01812 mg /L