Pragya tested the solubility of three different substances at different temperatures and collected the data as given below (results are given in the following table, as grams of substance dissolved in 100 grams of water to from a saturated solution)
Substance Dissolved - Temperature in K
283, 293, 313, 333, 353
Potassium nitrate 21, 32, 62, 106, 167
Sodium chloride 36, 36, 36, 37, 37
Potassium chloride 35, 35, 40, 46, 54
Ammonium chloride 24, 37, 41, 55, 66
(a) What mass of potassium nitrate would be needed to produce a saturated solution of potassium nitrate in 50 grams of water at 313 K?
(b) Pragya makes a saturated solution of potassium chloride in water at 353 K and leaves the solution to cool at room temperature. What would she observe as the solution cools? Explain.
(c) Find the solubility of each salt at 293 K. Which salt has the highest solubility at this temperature?
(d) What is the effect of change of temperature on the solubility of a salt?
(a) Solubility of potassium nitrate at 313 K = 62/100
∵ 100 g of water contains potassium nitrate = 62 g
∴ 50 g water contains potassium nitrate = 62/100 x 50 = 31 g
Thus, 31 g potassium nitrate would be needed to produce a saturated solution of potassium nitrate in 50 g of water at 313 K.
(b) When a saturated solution of potassium chloride at 353 K is cooled, the solubility of potassium chloride in water decreases. As a result the amount of potassium chloride which exceeds its solubility at lower temperature separates out as crystals.
(c) The maximum amount of the salt which is dissolved in 100 g of water to form a saturated solution at the given temperature, is known as solubility.
Solubility of potassium nitrate at 293 K = 32/100
Solubility of sodium chloride at 293 K = 36/100
Solubility of potassium chloride at 293 K = 35/100
Solubility of ammonium chloride at 293 K = 37/100
From the above data, it is clear that ammonium chloride has the highest solubility at 293 K.
(d) In general, the solubility of a salt increases with temperature. The increase is, however, different for different salts. For example, the solubility of potassium nitrate increases appreciably, that of ammonium chloride increases slightly, that of potassium chloride increases marginally while that of sodium chloride almost remains constant.