Question

20.0 mL of HCl in a flask Titration required 12.3 mL of 0.108M Mg(OH)-2 What is the molarity of the HCl?

Answer 1

Mg(OH)2 + 2HCl ==> 2H2O + MgCl2

mols Mg(OH)2 = M x L = ?
mols HCl = twice that (Look at the coefficients in the balanced equation.)
M HCl = mols HCl/L HCl. You know mols and L, solve for M.