WAEC Chemistry 2010 Paper

Concept:For equal molar concentrations of strong electrolytes, the solution that gives the most ions per formula unit has the highest conductivity.Explanation:All these salts dissolve completely in water to form free ions.Compare the number of ions produced per formula unit.$\mathrm{NH_4NO_3 \rightarrow NH_4^+ + NO_3^-}$ gives 2 ions.$\mathrm{NaNO_3 \rightarrow Na^+ + NO_3^-}$ gives 2 ions.$\mathrm{Mg(NO_3)_2 \rightarrow Mg^{2+} + 2NO_3^-}$ gives 3 ions.$\mathrm{Al(NO_3)_3 \rightarrow Al^{3+} + 3NO_3^-}$ gives 4 ions.More ions in solution means better electrical conductivity.Since $\mathrm{Al(NO_3)_3}$ produces the highest number of ions, it has the highest conductivity.Answer:D. $\mathrm{Al(NO_3)_3(aq)}$