JAMB Chemistry 2018 Paper

Concept:Heating ammonium chloride produces two gases that spread at different rates because of their different molar masses.Explanation:When $\mathrm{NH_4Cl}$ is heated, it decomposes as follows:$$\mathrm{NH_4Cl(s) \rightarrow NH_3(g) + HCl(g)}$$Ammonia, $\mathrm{NH_3}$, has a molar mass of about $17\,\mathrm{g\,mol^{-1}}$.Hydrogen chloride, $\mathrm{HCl}$, has a greater molar mass of about $36.5\,\mathrm{g\,mol^{-1}}$.According to Graham's law, the gas with the smaller molar mass diffuses faster.Therefore, ammonia reaches the litmus paper before hydrogen chloride.Dissolved ammonia gives an alkaline solution, which turns the litmus paper blue.Answer:D. turn blue