What is the common ion effect?

Posted on

Silver chloride is a sparingly soluble salt.

AgCl(s) ⇌ Ag+(aq) + Cl–(aq)

Ksp = [Ag+(aq)] [Cl–(aq)] = 2.0 x 10-10 mol2 dm-6

If I try to dissolve silver chloride in a solution of sodium chloride rather than water (just go with me here), will it be more or less soluble? The chloride ion represents the common ion in this example and we can either use le Chatelier’s principle to answer this question qualitatively or we can calculate an answer.

According to le Chatelier, addition of NaCl will increase the chloride ion concentration so the position of equilibrium will shift to the left. More AgCl(s) will precipitate out. Adding chloride ions effectively reduces the silver ion concentration and the solubility of silver chloride.

Let’s put some numbers to this …

Calculate the solubility of silver chloride in (a) pure water and (b) 0.15 mol dm-3 NaCl(aq).

Silver chloride is 10,000 times less soluble in NaCl than it is in pure water – the common ion effect is significant!

Practice questions

  1. Calculate the solubility of strontium sulphate, SrSO4, in 0.1 mol dm-3 sodium sulphate, given that its solubility product is 4.0 x 10-7 mol2 dm-6.
  1. Magnesium fluoride is a sparingly soluble salt that is used in the production of enamels for the ceramics industry.

(a)  write an equation for the equilibrium that is established when magnesium fluoride dissolves in water

(b)  predict and explain the effect on the equilibrium of adding aqueous sodium fluoride to the system

(c)  calculate the solubility of magnesium fluoride, MgF2, in 0.2 mol dm-3 sodium fluoride, given that its solubility product is 7.2 x 10-9 mol3 dm-9.

Answers

1.

2. (a)    MgF2(s)  ⇌  Mg2+(aq)  +  2F–(aq)

(b)   Adding sodium fluoride to the system adds a common ion, F–. An increase in the fluoride ion concentration causes the position of equilibrium to shift to the left to oppose the change (le Chatelier’s principle). The concentration of fluoride ions is clearly increased, the concentration of magnesium ions decreases and more magnesium fluoride will precipitate out of solution.