Chemistry Notes

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Chemistry Unit 2
Area of Study: 1: Water
Chapter 11: Measuring solubility
Measuring Solubility
Solubility: the maximum amount of that substance that can be dissolved in a that temperature Saturated solution: a solution which no more solute can be dissolved at that temperature Measuring solubility

Determine the maximum mass of solute that can be dissolved in 100 grams of solvent at a particular temperature
Worked Example
A maximum of a 6g of solute can be dissolved in 20g of water at 20ºC. What is the solubility of this solute in water at 20ºC?
6g of solute in 20g of water is equivalent to 30g (that is 6 x 100/20g) of solute dissolving in 100g of water at 20ºC
The solubility of this solute is therefore 30g per 100g at 20ºC Solubility curve: the relationship between solubility and temperature that is represented on and graph in a curve
G of NaNO3 had already been added
Worked example
An 80g of NaNO3 is added to 200g of H2O at 20ºC. Use the solubility curve (page 207 figure11.1) to calculate how much more NaNO3 needs to be added to make the solution saturated with NaNO3 at 20ºC

From the solubility curve, at 20ºC a saturated solution of NaNO3 contains 85g of NaNO3 in 100g of H2O.
So, in 200g of water, mass of NaNO3 needed to make a saturated solution: = 2 x 85
= 170g
80 g of NaNO3 has already been added to 200g H2O
So the extra amount of NaNO3 needed:
= 170 – 80
= 90g
Crystallisation
Crystallisation: when a solute turns to form crystals
Sugar becomes less soluble as it cools in honey
o Sugar no longer stays dissolved and comes out of the solution as crystals Worked example
What happens if a solution containing 50g of potassium nitrate in 100g of water is allowed to cool from 40ºC to 20ºC?
The solubility curve of potassium nitrate shows that, until the solution drops to32ºC, the 50g of solute will remain in solution
At 32ºC, 50 g of potassium nitrate is just enough to saturate 100g of water. As soon as the temperature drops a little more,...
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