A 2.135 g sample of a commercial antacid tablet claiming to contain "calcium…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A 2.135 g sample of a commercial antacid tablet claiming to contain "calcium carbonate as the active ingredient" is ground to a powder and transferred to a flask. Exactly 40.00 mL of 0.500 M HCl is added, and the mixture is allowed to react completely, producing CO₂ gas that escapes.
After the reaction is complete, the excess HCl remaining in the flask requires 15.60 mL of 0.200 M NaOH to reach the titration end point.
(a) Write the balanced chemical equation for the reaction between CaCO₃ and HCl that occurs before the back-titration.
(b) Calculate the number of moles of HCl originally added to the flask.
(c) Determine the number of moles of HCl that remained unreacted before the back-titration.
(d) Using your results, compute the mass percent of CaCO₃ in the antacid tablet.
(e) State two experimental assumptions that must be satisfied for the calculated mass percent to be valid, and briefly justify each.