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Question

 Antacid tablets are used to neutralize stomach acid and relieve indigestion or acid reflux. Many commercial antacids contain magnesium hydroxide, Mg(OH)2, as the active ingredient. If one tablet of antacid contains 15 mg of Mg(OH)2, then calculate the number of tablets required to completely neutralize 5 mL of stomach acid if the stomach acid is 0.2 M HCl.

Answer 1

Neutralization reaction

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

Find mols HCl in 5 mls of stomach acid

5 mls HCl x 1 L / 1000 mls x 0.2 mols HCl / L = 0.001 mols HCl

Find mols Mg(OH)₂ needed to neutralize 0.001 mols HCl (use mol ratio in balanced equation)

0.001 mols HCl x 1 mol Mg(OH)₂ / 2 mols HCl = 0.0005 mols Mg(OH)₂ needed

Find mols of Mg(OH)₂ in each tablet (use molar mass of Mg(OH)₂ = 58.3 g/mol)

15 mg Mg(OH)₂ / tablet x 1g/1000 mg x 1 mol Mg(OH)₂ / 58.3 g = 0.000257 mols Mg(OH)₂ / tablet

Find number of tablets needed to provide 0.0005 mols Mg(OH)₂

0.0005 mols Mg(OH)₂ x 1 tablet / 0.000257 mols Mg(OH)₂ = 1.94 tablets = 2 tablets are needed