0.040 mol of \({{\text{(N}}{{\text{H}}_{\text{4}}}{\text{)}}_{\text{2}}}{\text{Ni (S}}{{\text{O}}_{\text{4}}}{{\text{)}}_{\text{2}}} \bullet {\text{6}}{{\text{H}}_{\text{2}}}{\text{O}}\) is dissolved in water to give \({\text{200 c}}{{\text{m}}^{\text{3}}}\) of aqueous solution. What is the concentration, in \({\text{mol}}\,{\text{d}}{{\text{m}}^{ - 3}}\), of ammonium ions?

A.     0.00040

B.     0.0080

C.     0.20

D.     0.40

D

This was the most difficult question on the paper (correctly answered by 25.23% of the candidates) but it was fair as it merely required candidates to multiply 0.040 by 10. Most candidates gave answer C, forgetting that one mole of the compound contains two moles of ammonium ions. Candidates sometimes find the questions on Topic 1 somewhat testing and they could be advised to leave them until later in the given time.