Gravimetric exercises (2005 VCE)

The amount of calcium carbonate (CaCO3; molar mass = 100.1 g mol–1) in the ore dolomite can be determined by gravimetric analysis. The dolomite sample is dissolved in acid and the calcium ions (Ca2+) present are precipitated as calcium oxalate (CaC2O4; molar mass = 128.1 g/mol–). The calcium oxalate is filtered, dried and strongly heated to form calcium oxide (CaO; molar mass = 56.1 g/ mol–).

In one analysis the mass of dolomite used was 3.72 g. The mass of calcium oxide formed was found to be
1.24 g.
The percentage of calcium carbonate in the dolomite sample is closest to
A. 26.9
B. 33.3
C. 56.0
D. 59.5

Two possible sources of error in this analysis are
I – the precipitate of calcium oxalate is not rinsed with water after being Þ ltered.
II – the calcium oxide is not heated to constant mass.
Which of these two errors, if any, would lead to a result that is too high?
A. I only
B. II only
C. both I and II
D. neither I nor II