How many grams of HCl can a 200 mg dose of Mg(OH)2 neutralize?

Question

Antacids are bases that neutralize acids in the digestive tract. Magnesium hydroxide [Mg(OH)₂] is one such antacid. It reacts with hydrochloric acid in the stomach according to the following reaction:

Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O

How many grams of HCl can a 200 mg dose of Mg(OH)₂ neutralize?

Answer 1

Mg(OH)2 + 2HCl ==> MgCl2 + 2H2O ... balanced equation

Find moles of Mg(OH)2 used, then moles of HCl that it can neutralize using the stoichiometric ratios. Finally, convert moles HCl to grams HCl.

moles Mg(OH)2 used = 200 mg x 1 g/1000 mg x 1 mol/58.32 g = 0.003429 moles

moles HCl neutralized = 0.003429 mol Mg(OH)2 x 2 moles HCl/mol Mg(OH)2 = 0.006859 moles HCl

grams HCl = 0.006859 moles x 36.5 g/mole = 0.25 grams HCl = 0.3 g (to 1 sig. fig.)