Determination of solubility :

The solubility of salts is generally determined by gravimetric method. First of all a saturated solution is prepared. Some part of this saturated solution is weighed out in a porcelain dish. The solution is evaporated slowly to dryness on a sand bath. The dish is cooled and weighed again. The observations are recorded as follows:

                             Alt text : solubility of salts by gravimetric method

 

Alt text :

1.   Mass of empty dish = w g

 

2.   Mass of dish + solution = w₁ g

 

3.   So  Mass of solution = (w₁ – w) g

 

4.   Mass of dish + residue = w₂ g

 

So     Mass of residue = (w₂ – w) g = x g

 

and   Mass of solvent = (mass of solution – mass of residue)

= (w₁ – w) – (w₂ – w)

 

= (w₁ – w₂) = y g

 

Thus, the solubility of salt = x/y × 100 g per 100 g of solvent.

 

Example:          50 g of a saturated aqueous solution of potassium chloride at 30°C is evaporated to dryness, when 13.2 g of dry KCl was obtained. Calculate the solubility of KCl in water at 30°C.

Solution:           Mass of water in solution = (50 – 13.2) = 36.8 g

 

Solubility of KCl = Mass of KCl/Mass of water × 100 = 13.2/36.8 × 100

 

 

Mass of water present in dilute solution

To saturate 100 g of water, CuSO₄ required = 25 g

So, to saturate 88 g of water, CuSO₄ required 25/100 × 88

                      = 22 g