First set-up a balanced chemical equation:
2HCl + Mg(OH)2 ---> MgCl2 + 2H2O
Given: 30mL of 5.6M HCl and excess of Mg(OH)2
To determine the amount of magnesium chloride produced:
MgCl2 (g) = (30mL * (1L/ 1000mL) * 5.6 mol/L ) * (1 mol MgCl2/ 2 mol HCl) * (94.9 g/ mol MgCl2)
MgCl2 (g) = 7.97 g