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 K_sp= 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.

Answer 1

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.