What will be the concentration of sulfate ions in \({\text{mol}}\,{\text{d}}{{\text{m}}^{ - 3}}\) when 0.20 mol of \({\text{KAl(S}}{{\text{O}}_{\text{4}}}{{\text{)}}_{\text{2}}}\) is dissolved in water to give \({\text{100 c}}{{\text{m}}^{\text{3}}}\) of aqueous solution?

A.     0.2

B.     1.0

C.     2.0

D.     4.0

D

One respondent stated that general chemical nomenclature of ionic species is not explicitly on the syllabus and that a compound such as \({\text{KAl(S}}{{\text{O}}_{\text{4}}}{{\text{)}}_{\text{2}}}\) is not encountered frequently. For this reason, the respondent felt that students would have found this individual question difficult. The question overall proved to be the second most difficult question on the entire paper (after Question 5), with an associated Difficulty Index of 29%. However, it also proved to be a good discriminator with a value of 0.50. The question itself is based on AS 1.5.2 and has nothing to do with chemical nomenclature. As the formula of the species is given in the question, the most common mistake for students related to the fact that there are two sulfates present, leading to an answer of 4.0 and not 2.0. As an aside in response to the G2 comment itself, the syllabus does state in AS 4.1.7 that the formula of common polyatomic ions formed by non-metals in periods 2 and 3 should be known and a number of such species are listed in the TN corresponding to this AS, which includes sulfate.