WAEC Chemistry 2012 Paper

Concept:Mercury(I) exists as the dimeric ion $\mathrm{Hg}_2^{2+}$, and dioxonitrate(III) is the nitrite ion $\mathrm{NO}_2^-$.Explanation:In mercury(I) compounds, two mercury atoms are bonded together, so the ion is $\mathrm{Hg}_2^{2+}$ with a charge of $+2$.Dioxonitrate(III) contains the nitrite ion $\mathrm{NO}_2^-$, whose nitrogen is in the $+3$ oxidation state and carries a $-1$ charge.To form a neutral compound, one $\mathrm{Hg}_2^{2+}$ ion must combine with two $\mathrm{NO}_2^-$ ions.Therefore, the correct formula shows the mercury group once and the nitrite group twice, enclosed in brackets.Answer:$\mathrm{Hg}_2(\mathrm{NO}_2)_2$ — Option C.