Calcium is determined gravimetrically by precipitating it as CaC2O4â‹… H2O, followed by isolating the precipitate as CaCO3. The sample to be analyzed is dissolved in 10 mL of water and 15 mL of 6 M HCl. After dissolution, the resulting solution is heated
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to boiling, and a warm solution of excess ammonium oxalate is added. The solution is maintained at 80 °C, and 6 M NH3 is added dropwise, with stirring, until the solution is faintly alkaline. The resulting precipitate and solution are removed from the heat and allowed to stand for at least 1 h. After testing the solution for completeness of precipitation, the sample is filtered, washed with 0.1% w/v ammonium oxalate, and dried at 100–120 °C for 1 h. The precipitate is then transferred to a muffle furnace where it is converted to CaCO3 by drying at 500 ± 25 °C until a constant weight. (a) Why is the precipitate of CaC2O4⋅ H2O converted to CaCO3? (b) In the final step, if the sample is heated at too high of a temperature, some CaCO3 may be converted to CaO. What effect would this have on the reported %w/w Ca? (c) Why is the precipitant, (NH4)2C2O4, added to a hot, acidic solution rather than to a cold, alkaline
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