Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate, CaCO3. It forms under relatively deep marine conditions from the gradual accumulation of minute calcite plates (coccoliths) shed from micro-organisms called coccolithophores. A sample of chalk was analyzed for its calcium carbonate content. The chalk was ground to a fine

Question

powder and a 0.2996 g sample was placed in a 250 mL Erlenmeyer flask. Exactly 50 mL of a 0.1525 molar solution of hydrochloric acid was pipeted into the flask, whereupon the chalk dissolved with considerable foaming as the following reaction occurred.
CaCO3 (s) + 2 HCl �¨ CaCl2 (aq) + H2O + CO2 (g) After the foaming stopped, the result was a homogeneous solution that was acid, because excess HCl was added. A drop of phenolphthalein was added and the excess HCl was titrated with a 0.1122 molar solution of sodium hydroxide. The titration required 39.02 mL NaOH solution to neutralize the excess acid. What was the weight percent of calcium carbonate in the chalk?

Answer 1

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