Question

How many grams of magnesium hydroxide do you need in an antacid tablet to neutralize 20 mL of stomach acid? (Stomach acid is 0.1M HCl. There are 3.65 g of HCl in 1 L of stomach acid.) Show your work.

Answer 1

1 L of acid has 3.65 g HCl

that is

1000 mL of acid has 3.65 g HCl

So,

20 mL will have 3.65*20/1000 = 0.073 g

Molar mass of HCl = 1*MM(H) + 1*MM(Cl)

= 1*1.008 + 1*35.45

= 36.458 g/mol

mass of HCl = 0.073 g

mol of HCl = (mass)/(molar mass)

= 0.073/36.458

= 0.0020023 mol

we have the Balanced chemical equation as:

2HCl Mg(OH)2 —> MgCl2 + 2 H2O

From balanced chemical reaction, we see that

when 2 mol of HCl reacts, 1 mol of Mg(OH)2 is formed

mol of Mg(OH)2 formed = (1/2)* moles of HCl

= (1/2)*0.0020023

= 0.0010012 mol

Molar mass of Mg(OH)2 = 1*MM(Mg) + 2*MM(O) + 2*MM(H)

= 1*24.31 + 2*16.0 + 2*1.008

= 58.326 g/mol

mass of Mg(OH)2 = number of mol * molar mass

= 0.0010012*58.326

= 0.0584 g

Answer: 0.0584 g