Answer the following questions with the help of the data given in the table:
Table: Solubility of various salts (in g per 100 g of water) at different temperatures
| Salts | Temperature (°C) | |||||
|---|---|---|---|---|---|---|
| 10 °C | 20 °C | 30 °C | 40 °C | 60 °C | 80 °C | |
| Potassium nitrate | 21 | 32 | 45 | 62 | 106 | 167 |
| Sodium chloride | 36 | 36 | 36.3 | 36.5 | 37 | 37 |
| Potassium chloride | 35 | 35 | 37.4 | 40 | 46 | 54 |
| Ammonium chloride | 24 | 37 | 41 | 41 | 55 | 66 |
(A) What mass of potassium nitrate would be needed to prepare its saturated solution in 50 g of water at 40 °C?
(B) A student makes a saturated solution of potassium chloride in water at 80 °C and leaves the solution to cool at room temperature (25 °C). What would she observe as the solution cools? Explain.
(C) What is the effect of a change in temperature on the solubility of salts? Also, compare the changes in the solubility of the four given salts with increasing temperature from 10 °C to 80 °C.
(A) According to the data, the solubility of potassium nitrate at 40 °C is 62 g per 100 g of water. This means that 100 g of water can dissolve 62 g of potassium nitrate at this temperature.
Therefore, the mass of potassium nitrate needed for 50 g of water at 40 °C:
Hence, 31 g of potassium nitrate is needed to prepare a saturated solution in 50 g of water at 40 °C.
(B) As the solution cools, crystals of potassium chloride will separate out. This happens because the solubility of potassium chloride decreases at lower temperatures, so the excess dissolved salt crystallises from the solution.
(C) The solubility of most solid salts in water increases with an increase in temperature, although the extent of increase depends on the nature of the salt.
From 10 °C to 80 °C, potassium nitrate shows the maximum increase in solubility, from 21 g to 167 g per 100 g of water. Ammonium chloride also increases considerably, from 24 g to 66 g. Potassium chloride shows a gradual increase, from 35 g to 54 g, while sodium chloride shows almost negligible change, from 36 g to 37 g. Therefore, the order of increase in solubility from 10 °C to 80 °C is: Potassium nitrate > Ammonium chloride > Potassium chloride > Sodium chloride
