Neco 2020 Chemistry Question 4

A student carries out a gravimetric analysis to determine the purity of a limestone sample (calcium carbonate). The procedure is as follows: 1. Accurately weigh 5.00 g of the powdered limestone and transfer it to a 250 mL conical flask. 2. Add excess dilute hydrochloric acid to react the calcium carbonate according to: \(\mathrm{CaCO_3 + 2HCl \rightarrow CaCl_2 + CO_2(g) + H_2O}\). 3. The carbon dioxide gas produced is collected over water at 25 °C. The volume of gas measured is 0.845 L. 4. The water vapour pressure at 25 °C is 23 mm Hg and the atmospheric pressure is 760 mm Hg. 5. The remaining solution is evaporated to dryness and the solid residue is heated strongly in a crucible to convert any calcium carbonate to calcium oxide: \(\mathrm{CaCO_3 \xrightarrow{\Delta} CaO + CO_2}\). The mass of the final CaO residue obtained is 0.702 g. Answer the following: a) Calculate the number of moles of \(\mathrm{CO_2}\) collected. b) Determine the mass of calcium carbonate originally present in the sample. c) Calculate the percent purity of the limestone sample with respect to calcium carbonate. d) Using the mass of CaO obtained, calculate the experimental percent yield of CaO based on the theoretical amount calculated from part (b). Comment on two possible sources of error that could account for the difference between the experimental and theoretical yields.