Doc Brown's Chemistry  GCSE/IGCSE Science-Chemistry Revision Notes

See also Advanced Level Chemistry Students Acid-Base Revision Notes - use index

8a. A Summary of important formulae, solubility and water of crystallisation

Other than the oxides, hydroxides and carbonates (1st column), the original acids are

hydrochloric acid HCl,   sulphuric/sulfuric acid H₂SO₄  and  nitric acid HNO₃

which give the salts (2nd column) when reacted with a metal, oxide, hydroxide or carbonate.

Note that salt preparations are described in section 6

The negative ions (anions): O^2- oxide (O), OH^- hydroxide (OH), CO₃^2- carbonate (CO₃), HCO₃^- hydrogencarbonate (HCO₃), Cl^- chloride (Cl), SO₄^2-  sulfate (SO₄), NO₃^- nitrate (NO₃)

Assume the compounds are soluble unless otherwise stated.

The 3rd right column shows what ion/metal can be 'substituted' into the formulae in the 1st/2nd columns.

QUITE SIMPLY YOU SUBSTITUTE M with Li, Na, K, Ca, Mg, Fe, Cu, Zn, Al etc. TO GET THE FORMULA

The valency is the numerical chemical combing power of an atom/ion.

How to work out formulae is explained on another web page

Note that salt preparations are described in section 6

8b Solubility of salts - solubility curves

• 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.

8c. Water of crystallisation calculations

• How do we determine the water of crystallisation of a salt?

• Given data, how do calculate the water of crystallisation of a salt?

• Determination and calculation of salt formula containing 'water of crystallisation'.

  • Some salts, when crystallised from aqueous solution, incorporate water molecules into the structure. This is known as 'water of crystallisation', and the 'hydrated' form of the compound.

  • e.g. magnesium sulphate MgSO₄.7H₂O. The formula can be determined by a simple experiment (see the copper sulphate example below).

  • A known mass of the hydrated salt is gently heated in a crucible until no further water is driven off and the weight remains constant despite further heating. The mass of the anhydrous salt left is measured. The original mass of hydrated salt and the mass of the anhydrous salt residue can be worked out from the various weighings.

  • The % water of crystallisation and the formula of the salt are calculated as follows:

    • Suppose 6.25g of blue hydrated copper(II) sulphate, CuSO₄.xH₂O, (x unknown) was gently heated in a crucible until the mass remaining was 4.00g. This is the white anhydrous copper(II) sulphate.

    • The mass of anhydrous salt = 4.00g, mass of water (of crystallisation) driven off = 6.25-4.00 = 2.25g

    • The % water of crystallisation in the crystals  is 2.25 x 100 / 6.25 = 36%

    • [ A_r's Cu=64, S=32, O=16, H=1 ]

    • The mass ratio of CuSO₄ : H₂O is 4.00 : 2.25

    • To convert from mass ratio to mole ratio, you divide by the molecular mass of each 'species'

    • CuSO₄ = 64 + 32 + (4x18) = 160 and H₂O = 1+1+16 = 18

    • The mole ratio of CuSO₄ : H₂O is 4.00/160 : 2.25/18

    • which is 0.025 : 0.125 or 1 : 5, so the formula of the hydrated salt is CuSO₄.5H₂O

• All concentration calculations are covered on the on-line calculations page, especially sections 7. on molarity, 11. and 12. on molarity and acid-base (alkali) titrations, section 14.3 on dilutions.

• Note that salt preparations are described in section 6

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• Multiple choice revision quizzes and other worksheets

• GCSE/IGCSE foundation-easier multiple choice quiz on pH, Indicators, Acids, Bases, Neutralisation and Salts

• GCSE/IGCSE higher-harder multiple choice quiz on pH, Indicators, Acids, Bases, Neutralisation and Salts

• GCSE/IGCSE Structured question worksheet on Acid Reaction word equations and symbol equation questions

•  Word equation answers and symbol equation answers)

• GCSE/IGCSE word-fill worksheet on Acids, Bases, Neutralisation and Salts

• GCSE/IGCSE matching pair quiz on Acids, Bases, Salts and pH

• See also Advanced Level Chemistry Students Acid-Base Revision Notes - use index