A student compares two different laboratory procedures for determining the amount of…
CHEMISTRY · Multi-part · A Level · Chemistry · Amount of substance · Amount of substance
Problem
A student compares two different laboratory procedures for determining the amount of calcium carbonate in a seashell sample.
• In Procedure 1 the shell is dissolved in excess hydrochloric acid, and the volume of carbon dioxide produced is measured directly in an inverted graduated cylinder at 293 K and 1.00 atm.
• In Procedure 2 the shell is dissolved in the same manner, but the carbon dioxide is first passed through a drying tube, cooled to 273 K, and then its pressure is adjusted to 1.00 atm before its volume is measured.
Assume that in both procedures the hydrochloric acid is present in large excess and that no other gases are produced.
(a) Describe why writing a correctly balanced chemical equation for the reaction between calcium carbonate and hydrochloric acid is essential before attempting any quantitative analysis of the experimental data.
(b) In Procedure 1 the concentration of hydrochloric acid is doubled while the mass of the seashell remains the same. Explain qualitatively how, if at all, the measured volume of carbon dioxide would change and justify your reasoning.
(c) In Procedure 2 an error occurs and the carbon dioxide is accidentally measured at 283 K instead of the intended 273 K while the pressure remains at 1.00 atm. Explain how this temperature error would affect the calculated number of moles of carbon dioxide and the subsequent determination of the mass percent of calcium carbonate in the shell.