A student took 100 mL of a solution of Mg(OH)2 and reacted it with HCl (aq) to calculate the concentration of hydroxide ions in the solution. If 13.75 mL of 0.010M HCl was used to react with all of the OH- (aq) in the 100-mL solution, what was the concentration of hydroxide ions dissolved in the 100-mL solution?

Question

A student took 100 mL of a solution of Mg(OH)2 and reacted it with HCl (aq) to calculate the concentration of hydroxide ions in the solution.  If 13.75 mL of 0.010M HCl was used to react with all of the OH- (aq) in the 100-mL solution, what was the concentration of hydroxide ions dissolved in the 100-mL solution?

DrBob222 · Answer

Mg(OH)2 + 2HCl ==> MgCl2 + 2H2O
mols HCl = M x L = ?
mols Mg(OH)2 = 1/2 that (use the coefficients in the balanced equation.)
M Mg(OH)2 = mols Mg(OH)2/L Mg(OH)2.
Then M OH^-) = twice M Mg(OH)2 because there are two OH^- ions/Mg(OH)2.