# GCSE Chemistry - Key Calculations for C2 and C3

Clearly laid out calculations, using secondary sources.

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• Created by: Arjun
• Created on: 22-06-11 09:44

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14.4 Water of crystallisation
%6. Example 14.4.1: Calculate the % of water in hydrated magnesium
sulphate MgSO4.7H2O
%6. Relative atomic masses: Mg = 24, S = 32, O = 16 and H = 1
%6. relative formula mass = 24 + 32 + (4 x 16) + [7 x (1 + 1 + 16)] =
246
%6. 7 x 18 = 126 is the mass of water
%6. so % water = 126 x 100 / 246 = 51.2%
%6. Example 14.4.2 The % water of crystallisation and the formula of
the salt are calculated as follows:
%6. 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.
%6. The mass of anhydrous salt = 4.00g, mass of water (of
crystallisation) driven off = 6.254.00 = 2.25g
%6. The % water of crystallisation in the crystals is 2.25 x 100 / 6.25 =
36%
%6. [ Ar's Cu=64, S=32, O=16, H=1 ]
%6. The mass ratio of CuSO4 : H2O is 4.00 : 2.25
%6. To convert from mass ratio to mole ratio, you divide by the
molecular mass of each 'species'
%6. CuSO4 = 64 + 32 + (4x18) = 160 and H2O = 1+1+16 = 18
%6. 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

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TITRATION CALCULATIONS
The question:
10.0 cm3 of a solution of potassium hydroxide was titrated with a 0.10 M[1]
solution of hydrochloric acid. 13.5 cm3 of the acid was required for neutralization.
Calculate the concentration of the potassium hydroxide solution.
Step 1
Write down everything you know. Placing the one you know most about on the left
(you don't have to do this, but its just a good idea).
Hydrochloric Acid Potassium hydroxide
Volume used = 13.5 cm3 Volume used = 10.0 cm3
Concentration = 0.…read more

### Page 3

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Number of moles of HCl = 0.0014 mol
Number of moles KOH = 0.0014 mol ­ they react in a one to one ratio
Step 4
Work out the concentration of the potassium hydroxide.
Concentration = 0.14 M
Questions
1. Sodium ethanedioate, Na2C2O4, can be made into a solution of
an exact and reliable concentration and so can be used as a
standard solution for checking the concentration of a solution
containing sulphuric acid.…read more