What volume of sulfur trioxide, in cm3, can be prepared using \({\text{40 c}}{{\text{m}}^{\text{3}}}\) sulfur dioxide and \({\text{20 c}}{{\text{m}}^{\text{3}}}\) oxygen gas by the following reaction? Assume all volumes are measured at the same temperature and pressure.

\[{\text{2S}}{{\text{O}}_2}{\text{(g)}} + {{\text{O}}_2}{\text{(g)}} \to {\text{2S}}{{\text{O}}_3}{\text{(g)}}\]

A.     20

B.     40

C.     60

D.     80

B

There were a number of comments that these questions had a high degree of commonality and that maybe other aspects of stoichiometry could have been tested. There were also a number of comments that these questions were testing mathematical ability rather than chemistry. Almost all of the calculations involved only required order of magnitude estimates to determine the correct response and hence did not require detailed mental arithmetic. In general the questions were tackled well with the mean difficulty index being about 56% and they also proved good discriminators with a mean discrimination index of \( > 0.5\).