Question 1
Different quantities of nitrogen oxide (NO) are listed below.
Which one contains the least number of molecules?
A. 6 × 10² L at 273 K and 1 atm
B. 6 × 10²³ molecules
C. 6 × 10² g
D. 6 mol

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Question 2
When 2.54 g of solid iodine reacts with excess chlorine and the unreacted chlorine is evaporated, 4.67 g of a yellow product remains.
The empirical formula of the product is
A. ICl₂
B. ICl₃
C. ICl₄
D. ICl₅

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Some rocks were thought to consist of insoluble silica (SiO₂) and calcium carbonate ( CaCO₃; molar mass 100.1g mol¹). The fraction of CaCO₃ in an 8.64 g sample of the crushed rock was determined by mixing the sample
with excess hydrochloric acid. The acid reacts with CaCO₃ according to the following equation.
2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g)
The resulting solution was filtered and the SiO₂ that was collected was washed and dried. The mass of SiO₂
was found to be 1.55 g.

a. Calculate the expected percentage of CaCO₃ in the original rock sample.

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b. In order to check the result from part a., excess ammonium oxalate solution was added to the filtered
solution. The calcium ions present precipitate as CaC₂O₄.H₂O
The CaC₂O₄.H₂O was collected by filtration, washed and dried. It was then heated to convert it to CaO
(molar mass 56.1 g mol¹) and a mass of 3.87 g was obtained.
Using this mass of CaO, calculate the percentage of CaCO₃ in the rock sample. Ensure that you express
your answer to an appropriate number of significant figures.

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c. Suppose that the percentage of CaCO₃ determined by the chemical analysis with ammonium oxalate was
less than the result found in part a. above. Provide one possible explanation for the difference.

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