A sample of an antacid tablet was analyzed by the following procedure, to determine how many moles of simulated stomach acid (0.131 M HCl) it would neutralize.

A 0.19 g sample of an antacid tablet was placed in an Erlenmeyer flask containing 20.00 mL of 0.131 M HCl. The mixture was stirred to facilitate the reaction between the acid and the antacid tablet sample.

The balanced chemical equation for the reaction of the main ingredient of the antacid tablet (CaCO₃) with the HCl is as follows.

        2 HCl(aq) + CaCO₃(s) -----> CaCl₂(aq) + H₂O(l) + CO₂(g)

The mixture was then titrated with 0.111 M NaOH. The volume of NaOH required to reach a phenolphthalein endpoint was 8.40 mL.

The balanced chemical equation for the reaction of HCl with NaOH is as follows.

        HCl(aq) + NaOH(aq) -----> NaCl(aq) + H₂O(l)

How many moles of HCl were neutralized by the antacid sample?

SOLUTION
HCl Total – HCl Titrated with NaOH = HCl Neutralized by antacid
 

HCl Total
(0.02000 L HCl)(0.131 M) = 2.62 x 10^-3 mol HCl
 

HCl Titrated with NaOH
(0.00840 L NaOH)(0.111 M) = 9.32 x 10^-4 mol NaOH

(9.32 x 10^-4 mol NaOH)(1 mol HCl / 1 mol NaOH) = 9.32 x 10^-4 mol HCl
 

HCl Neutralized by antacid
2.62 x 10^-3 mol HCl - 9.32 x 10^-4 mol HCl = 1.69 x 10^-3 mol HCl