Question

If 1.85 g of Mg(OH)2 reacts with 3.71 g of HCl how much MgCl2 is produced

Answer 1

\[Mg(OH) _{_{2}}+2HCl \rightarrow MgCl _{2}+H_{2}O\]
1 mole of Magnesium hydxide will 2 moles of Hydrochloric acid to give 1 mole of Magnesium Chloride

Answer 2

You have to convert the grams of Mg(OH)2 into moles
\[\large 1.85 g Mg(OH)_2 * \frac{1 mol}{58.32 g} = 0.032 mol \]

Like Narad stated, it's a 1:1 for Mg(OH)2 and MgCl2, so it would also produce 0.032 mol of MgCl2. Just multiply 0.032 mol by the mass.
\[\large 0.032 mol * \frac{95.211 g MgCl_2}{1 mol} = 3.02 g MgCl_2\]
So 3.02g of MgCl2 is produced in the reaction.