- Solubility graphs represent solubility in g per 100 g of water plotted against temperature.
- To plot a solubility curve, the maximum mass of solvent that can be dissolved in 100 g of water before a saturated solution is formed, is determined at a series of different temperatures.
Solubility graph for salts:
Example question 1:
How much potassium nitrate will dissolve in 20g of water at 34 °C?
At 34 °C the solubility is 49g per 100g of water
So scaling, 49 x 20 / 100 = 9.8 g of potassium nitrate will dissolve in 20 g of water.
Example question 2:
200 cm3 of saturated copper solution was prepared at a temperature of 90 °C. What mass of copper sulphate crystals form if the solution was cooled to 20 °C?
Solubility of copper sulphate at 90 oC is 67g/100g water, and 19g/100g water at 20 °C.
Therefore for mass of crystals formed = 67 – 19 = 48g (for 100 cm3 of solution).
However, 200 cm3 of solution was prepared,
So total mass of copper sulphate crystallised = 2 x 48 = 96g
Solubility graph for gases:
Unlike salts, shown in the previous graph, gases become less soluble as temperature increases. E.g. fizzy drinks become flat more quickly when left at a warmer temperature.