Question

Pragya tested the solubility of four 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 form 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?

Accepted Answer

(a) Since $62 g$ of potassium nitrate is dissolved in $100 g$ of water to prepare a saturated solution
Mass of potassium nitrate (solute) $= 62 g$
Mass of water (solvent) $= 100 g$
New mass of water $= \frac{100 g}{2} = 50 g$
Therefore, mass of potassium nitrate $= \frac{62 g}{2} = 31 g$
We therefore need $31 g$ of potassium nitrate to be dissolved in $50 g$ of water to prepare a saturated solution at $313 K .$

(b) The amount of potassium chloride that can be dissolved in water to make a saturated solution increases with temperature. As the solution cools, some of the crystals of potassium chloride will precipitate out of the solution as the amount of salt that is soluble at a lower temperature is less than that at a higher temperature.

(c) The solubility of the salts at $293 K$ are:
Potassium nitrate - $32 g$
Sodium chloride - $36 g$
Potassium chloride - $35 g$
Ammonium chloride - $37 g$
Ammonium chloride has the highest solubility at $293 K$ .

(d) Solubility is specific to solvents. In the case of liquids, the temperature must also be mentioned as the solubility varies with temperature. Solubility of most solid substances can be increased with a rise in the temperature. Therefore, at lower temperatures, the rate of solubility of a salt decreases while the rate of solubility of a salt increases with increase in temperature.

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