Question

Please explain when two ions can be separated effectively by this fractional precipitation.

Example:

AgNO3(aq) is slowly added to a solution that is 0.250 M NaCl and also 2.3×10⁻³ M KBr.

Can the Cl− and Br− be separated effectively by this fractional precipitation?

Answer 1

If two salts (AgCl and AgBr) which are formed by addition of precipitating agent (AgNO3) have different Ksp values then they can be separated effectively by this fractional precipitation method. That means the difference in solubility products Ksp values is larger then the ions can be separated more effectively. If the difference in solubility products values is less then co-precipitation takes place instead of fractional precipitation.

Ksp of AgBr is 7.7*10^-13 and Ksp of AgCl is 1.8*10^-10 these two values are closer so AgCl starts to precipitate before complete precipitation of AgBr. So These two ions can't be separated effectively by this fractional precipitation.