Gravimetric exercises (2005)
The following information refers to questions 1 and 2. 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 mol1-). The calcium oxalate is filtered, dried and strongly heated to form calcium oxide (CaO; molar mass = 56.1 g mol1-).


Question 1
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

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Question 2
Two possible sources of error in this analysis are
I - the precipitate of calcium oxalate is not rinsed with water after being filtered.
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

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Question 3
Hydrogen and chlorine react according to the equation
H2(g) + Cl2(g) → 2HCl(g)
3 mole of H2 and 2 mole of Cl2 are placed in a vessel and sealed.
When reaction is complete the vessel will contain
A. 5 mole of HCl
B. 6 mole of HCl and 1 mole of Cl2
C. 4 mole of HCl and 1 mole of Cl2
D. 4 mole of HCl and 1 mole of H2

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Question 4
An organic compound is known to contain only carbon, hydrogen and oxygen. The compound contains, by mass, 39.1% of carbon and 8.7% of hydrogen.
The number of carbon atoms in the empirical formula is
A. 1
B. 2
C. 3
D. 4

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