' $R$ ' is sodium sulphide. When it heated with. dil. $\mathrm{H}_2\mathrm{SO}_4$ a colourless gas $\mathrm{H}_2\mathrm{S}$ is released which turns moist lead acetate paper to silvery black, due to the formation of lead sulphide. $\;\mathrm{Na}_2\mathrm{S}_{\text{'R'}} + \;\mathrm{H}_2\mathrm{SO}_{\underset{\text{dil.}}{4}} \rightarrow \;\mathrm{Na}_2\mathrm{SO}_4 + \mathrm{H}_2\mathrm{S} \uparrow$ $(\mathrm{CH}_3\mathrm{COO})_2\mathrm{Pb} + \mathrm{H}_2\mathrm{S} \rightarrow \mathrm{PbS}_{\text{ Black }}$ $+ \mathrm{CH}_3\mathrm{COOH}$Anion is $\mathrm{S}^{2-}$ (Sulphide)