# GCSE Chemistry - Key Calculations for C2 and C3

Clearly laid out calculations, using secondary sources.

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

First 193 words of the document:

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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