Doc Brown's Chemistry Clinic SELECTED SALT SOLUBILITY DATA and calculation examples

for KS3 Science and GCSE-IGCSE chemistry courses

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• Interpretation of graph eg

  • Reading graph: at 38^oC the solubility of copper sulphate, CuSO₄, is 28g of anhydrous salt per 100g of water.

  • Reading graph: at 84^oC the solubility of potassium sulphate, K₂SO₄, is 22g per 100g of water.

  • Ex Q1: How much potassium nitrate will dissolve in 20g of water at 34^oC?

    • At 34^oC the solubility is 52g per 100g of water,

    • so scaling down, 52 x 20 / 100 = 10.4g will dissolve in 20g of water

  • Ex Q2: At 25^oC 6.9g of copper sulphate dissolved in 30g of water, what is its solubility in g/100cm³ of water?

    • Scaling up, 6.9 x 100 /30 = 23g/100g of water (check on graph, just less than 23g/100g water).

  • Ex Q3: 200 cm³ of saturated copper solution was prepared at a temperature of 90^oC. What mass of copper sulphate crystals form if the solution was cooled to 20^oC?

    • Solubility of copper sulphate at 90^oC is 67g/100g water, and 21g/100g water at 20^oC.

    • Therefore for mass of crystals formed = 67 - 21 = 46g (for 100 cm³ of solution).

    • However, 200 cm³ of solution was prepared,

    • so total mass of copper sulphate crystallised = 2 x 46 = 92g

• Note: The density of water is close to 1.0g/cm³ or ml, so for approximate purposes. the volume in cm³ or ml of just the water is numerically close to the value in g, i.e. 100 cm³ of water or solution is about 100g of water.

Examples of

SALT SOLUBILITY DATA

SOLUBILITY

g salt / 100g water

Salt name

potassium nitrate

potassium sulphate

sodium chloride

hydrated copper(II) sulphate

and formula

Temp. ^oC

KNO₃

K₂SO₄

NaCl

CuSO₄ (anhydrous *)

0

13.9

7.4

35.7

14.3

10

21.2

9.3

35.8

17.4

20

31.6

11.1

36.0

20.7

30

45.3

13.0

36.2

24.2

40

61.4

14.8

36.5

28.7

50

83.5

16.5

36.8

33.8

60

106.0

18.2

37.3

40.0

70

19.8

37.6

47.0

80

21.4

38.1

56.0

90

22.9

38.6

67.5

100

24.1

39.2

80.0

* multiply by 1.562 for hydrated crystals CuSO₄.5H₂O

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