Magnesium hydroxide reacts with hydrochloric acid to form magnesium chloride and water.
Mg(OH)2 + 2 HCl → MgCl2 + 2 H2O
This balanced equation, which is pretty typical of an acid-base reaction (acid + base → salt + water) is needed to figure out the mole ratio of acid to base. The mole ratio is "1 mol Mg(OH)2 = 2 mol HCl". You're given the volume (mL) and molarity (mol/L) of the acid, which can be used to find moles of acid. You need to find the grams of base, so it's a good idea to write up a conversion plan.
It's often a good idea to start your plan with the answer's units. To get grams of base, you'll need the moles of base. You can get the moles of base from the balanced equation and the moles of acid. You can get the the moles of acid if you know the volume in Liters and the molarity, which is mol/L. Volume in L can be found from the volume in mL.
Plan: mL HCl → L HCl → mol HCl → mol Mg(OH)2 → grams Mg(OH)2
To do the last conversion, you'll need to calculate the molar mass of magnesium hydroxide.
- molar mass = 24.31 + (2 x 16.00) + (2 x 1.008) = 58.326 g/mol
Now the conversion can be put together and solved. Don't round anything until the end, or the answer will probably be off by a bit. The molarity of the acid limits your final answer to two sig figs.
25 mL HCl x (1 L / 1000 mL) x (0.10 mol HCl / 1 L HCl) x (1 mol Mg(OH)2 / 2 mol HCl)
x (58.326 g Mg(OH)2 / 1 mol Mg(OH)2) = 0.073 grams Mg(OH)2