CHEMISTRY 31

Quiz 3 - Solutions

1. Using reactions, explain whether NH₄Cl is a base, a neutral compound, or an acid when it dissolves in water. (3 pts)

NH₄Cl is an ionic compound and 100% dissociates: NH₄Cl → NH₄⁺ + Cl^-

Cl^- will not react further (it is neutral as it cannot react with water to form HCl – a strong acid)

NH₄⁺ is the conjugate acid of a weak base and is acidic: NH₄⁺ ↔ + NH₃(aq) + H⁺

Solution is acidic

2. A solution contains chromate, CrO₄^2-, at 0.080 M, and chloride at 0.050 M and it is desired to separate the two ions. Both form sparingly soluble salts with Ag⁺. The K_sp values for Ag₂CrO₄ and AgCl are 1.12 x 10^-12 and 1.77 x 10^-10, respectively.

a) Calculate [Ag⁺] when each anion starts precipitating (2 calculations) and decide which anion precipitates out first. (4 pt)

K_sp(Ag₂CrO₄) = [Ag⁺]²[CrO₄^2-] = 1.12 x 10^-12 or [Ag⁺] = (1.12 x 10^-12/0.080)^0.5 = 3.7 x 10^-6 M

K_sp(AgCl) = [Ag⁺][Cl^-] = 1.77 x 10^-10 or [Ag⁺] = (1.77 x 10^-10/0.050) = 3.5 x 10^-9 M

Since AgCl requires a lower [Ag⁺], Cl^- precipitates first

b) Calculate the concentration of the first anion to precipitate when just enough Ag⁺ exists to start precipitating the second anion. (3 pts)

Since the 2^nd anion to precipitate is CrO₄^2-, [Ag⁺] = 3.7 x 10^-6 M (see a))

Now we can calculate [Cl^-] in equilibrium with that concentration.

[Cl^-] = (1.77 x 10^-10/3.7 x 10^-6) = 4.7 x 10^-5 M