U.S. National Chemistry Olympiad · 2024

Problems

Problem 1An unknown salt $\ce{MX2}$ is a group 2 metal halide. (a) 10.00 g of $\ce{MX2}$ dissolves in 50.0 g of water to give a homogeneous solution whose freezing point is $-4.50\ ^{\circ}\mathrm{C}$ . What is the molar mass of $\ce{MX2}$ ? For water, $K_f = 1.86\ ^{\circ}\mathrm{C}\,m^{-1}$ . (b) 10.00 g of $\ce{Na2CO3}$ and 10.00 g of $\ce{MX2}$ are mixed in 200.0 mL of water and a precipitate of $\ce{MCO3}$ forms. What is the pH of the supernatant? The $K_a$ of $\ce{H2CO3}$ is $4.3 \times 10^{-7}$ and the $K_a$ of $\ce{HCO3-}$ is $4.7 \times 10^{-11}$ . (c) A solution of 10.00 g of $\ce{MX2}$ in water is treated with excess silver nitrate; the dried precipitate has mass 15.2 g. What is the identity of $\ce{MX2}$ ? (d) A sample of 10.00 g of $\ce{MX2}$ dissolved in 50 mL of water is treated with increasing amounts of $\ce{Na2SO4}$ up to 10 g in total; describe how the mass of precipitate varies with the mass of added $\ce{Na2SO4}$ . (e) What colour flame test does $\ce{MX2}$ give?Solutions: 1