First, you write the balanced equation for the reaction
2HCl + Mg(OH)2 ----->MgCl2 + 2H2O
Next, find the number of moles of Magnesium Hydroxide
Number of moles of Mg(OH)2 =
mass / Mr = 3.33 / 24 +2(1+16)
=0.0574 mol
Find the mole ratio between HCl and Mg(OH)2 which is 1:2
Number of moles of HCl = 0.0574 x 2 = 0.1148 mol
Mass of HCl = Mr x no. of moles
=0.1148 x (1+35.5)
=4.19 g
