A question arises. Is the magnesium reacting with the water or is it reacting wi

Question

A question arises. Is the magnesium reacting with the water or is it reacting with the acid? The next part of the experiment will answer this question. Sodium hydrogen carbonate, NaHCO3, is both a weak acid and a weak base, and as a consequence its aqueous solutions are                   buffered. Moreover, the buffered pH is approximately 8 which corresponds to a slightly alkaline condition.   

Using the Ksp= 1.5 x 10-11 for Mg(OH)2 predict whether or not a solution of NaHCO3 should remove the Mg(OH)2 coating from magnesium.

Explanation / Answer

Magnesium reacts with acid in water. It acts both as acid and base.

Ksp of Mg(OH)2 = [Mg2+][OH-]^2

let x be the molar solubiluty of Mg(OH)2

1.5 x 10^-11 = (x)(2x)^2

                     = 4x^3

x = 5.05 x 10^-3 M

[OH-] = 2 x 5.05 x 10^-3 = 0.015 M

pOH = -log[OH-] = 1.82

pH = 14 - pOH = 12.18

So the pH of Mg(OH)2 is 12.18, whereas, the pH of NaHCO3 is 8. So Mg(OH)2 coating would not be removed from magnesium by NaHCO3.

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