Milk of magnesia has 400 mg of Mg (OH) 2 per teaspoon.

Question

Calculate the mass of stomach acid that can be neutralized by 3.90 teaspoon of milk of magnesia, assuming that the stomach acid is 0.50%  by mass.

Answer 1

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

3.90 tsp........? grams

3.90 tsp x 400 mg/tsp = 1560 mg Mg(OH)2

moles Mg(OH)2 = 1560 mg x 1 g/1000 mg x 1 mol Mg(OH)2/58.3 g = 0.0268 moles Mg(OH)2 used

moles HCl neutralized = 0.0268 mol Mg(OH)2 x 2 mol HCl/mol Mg(OH)2 = 0.0535 moles HCl

mass HCl neutralized = 0.0535 mol HCl x 36.5 g /mol = 1.95 g = 1950 mg HCl neutralized

The fact that it is 0.5% by mass does not enter into the calculations, unless you want to solve for the VOLUME of stomach acid.