Question

a) Calculate the solubility of silver chloride (AgCl) in a 6.5x10^-3 M silver nitrate (AgNO3) solution.
b) Which of the following compounds will be more soluble in acidic solution than in water:
a) CuS b) AgCl c) PbSO4 ?

Answer 1

AgNO3 ionizes as AgNO3 == Ag+ + NO3-

[Ag+] = [NO3-] = 6.5*10^-3 M
   [AgCl] <==> [Ag+] + [Cl-]
Initial         6.5x10^-3            0
Change           +x                 +x
Equilibrium 6.5x10^-3     x

Ksp = [Ag+][Cl-] / [AgCl]
2.5x10^-8 = (6.5x10^-3)(x)
x = 3.8x10^-6 M

mol/L to g/L
= (moles x mol.wt of AgCl)
143.32g/mol is the mw for AgCl
= (3.8x10^-6 mol / 1L) x (143.32g / 1mol) = 5.5x10^-5 g/L

b) A weak acid anion will combine with protons in an acidic solution much more easily. This will help stabilise the ion, and hence it will improve solubility of the salt.
S^2- is the anion of a weak acid. This makes S^2- a basic anion, capable of reacting with acids to form HS^- and H2S. It does so quite readily as well, which will dissolve the CuS in acidic solution
Thus CuS is more soluble in acid than in normal water