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A groundwater contains magnesium at a concentration of 182.0 mg/L as CaCO3. The magnesium is to...

A groundwater contains magnesium at a concentration of 182.0 mg/L as CaCO3. The magnesium is to be removed by precipitation as Mg(OH)2. What must the pH be in order to attain a final magnesium concentration of 6.0 mg/L as CaCO3? Note: Mg(OH)2 = Mg2+ + 2OH-, pKs = 11.25

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Answer #1

Dissolution equilibrium for magnesium hydroxide :

solubility product constant is given by

Concentration of oh ions will be calculated as :

since concentration of mg ion will be the difference of initial and final concentration which will give the concentration that needs to be precipitated.

also:

at pH of 13.4, final concentration of mg ions will reach to 6 mg/l.

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