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Brown's Chemistry  GCSE/IGCSE/GCE (basic A level)
O Level
Online Chemical Calculations
4.
Calculating the %
composition by mass of elements in compounds
This page describes,
and explains,
with fully worked out examples, how to calculate the composition of a
compound in terms of the % by mass of each element in a compound. It
doesn't matter what the nature of the compound is i.e. it is irrelevant
whether its an ionic compound or a covalent compound.
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4.
Method of calculating the %
percentage by mass of the elements in a compounds
The 'percent' % by mass composition of a compound in terms of its constituent elements is calculated
in three easy steps
Chemistry calculations 4.
How do you calculate the percent (%) by mass of
an element in a compound formula?
How do you calculate the percent (%) by mass of
water or an ion in a compound formula?
(i) Calculate the formula or molecular mass of the compound
see section 2.
2.
Calculating relative formula/molecular mass (M_{r}) of a compound
(ii) Calculate the mass of the specified element
(for its %) in the compound, taking into account the number of atoms of the element in the compound formula
(iii) Calculate (ii) as a percentage of (i)
 Calculation of % composition Example 4.1
 Calculate the % of copper in copper sulphate,
CuSO_{4}
 Relative atomic masses: Cu = 64, S = 32 and O = 16
 relative formula mass = 64 + 32 + (4x16) = 160
 only one copper atom of relative atomic mass 64
 % Cu = 64 x 100 / 160
 =
40% copper by mass in the compound
 Note that similarly, you can calculate the % of the
other elements in the compound e.g.
 % sulfur = (32/160) x 100 =
20% S
 % oxygen = (64/160) x 100 =
40% O
 Also note that if you haven't made any errors, they
should add up to 100% !
 Calculation of % composition Example 4.2
 Calculate the % of oxygen in aluminium sulphate,
Al_{2}(SO_{4})_{3
}
 Relative atomic masses: Al = 27, S = 32 and O = 16
 relative formula mass = 2x27 + 3x(32 + 4x16) = 342
 there are 4 x 3 = 12 oxygen atoms, each of relative atomic mass 16,
 giving a total mass of oxygen in the formula of 12 x 16 = 192
 % O = 192 x 100 / 342 =
56.1% oxygen by mass in aluminium sulphate

Calculation of % composition Example 4.3
 The next two examples extend the idea of %
element composition to include % composition of part of a compound, in these
cases water in a hydrated salt and the sulfate ion in a potassium salt.
 Calculate the % of water in hydrated
magnesium sulphate MgSO_{4}.7H_{2}O
 Relative atomic masses: Mg = 24, S = 32, O = 16 and H = 1
 relative formula mass = 24 + 32 + (4 x 16) + [7 x (1 + 1
+ 16)] = 246
 7 x 18 = 126 is the mass of water
 so % water = 126 x 100 / 246 =
51.2 % H_{2}O
 Note: The determination
and calculation of the formula of a hydrated salt like MgSO_{4}.7H_{2}O
is covered in Calculations section
14.4.

Calculation of % composition Example 4.4
 Calculate the percentage by mass, of sulfate
ion in sodium sulfate
 formula of sodium sulfate Na_{2}SO_{4},
atomic masses: Na = 23, S = 32, O = 16
 Formula mass Na_{2}SO_{4} =
(2 x 23) + 32 + (4 x 16) = 142
 Formula mass of sulfate ion SO_{4}^{2}
(or just SO_{4} will do for the calculation) = 32 + (4 x 16) = 96
 Therefore % sulfate ion in sodium sulfate =
(96/142) x 100 = 67.6% SO_{4}
Selfassessment Quizzes
F = easierfoundation, H = harderhigher (but does include F
questions)
[pbm] type in answer
for
F and H or
multiple choice
for
F and H
OTHER CALCULATION PAGES

What is relative atomic mass?,
relative isotopic mass and calculating relative atomic mass

Calculating relative
formula/molecular mass of a compound or element molecule

Law of Conservation of Mass and simple reacting mass calculations

Composition by percentage mass of elements
in a compound
(this page)

Empirical formula and formula mass of a compound from reacting masses
(easy start, not using moles)

Reacting mass ratio calculations of reactants and products
from equations
(NOT using
moles) and brief mention of actual percent % yield and theoretical yield,
atom economy
and formula mass determination

Introducing moles: The connection between moles, mass and formula mass  the basis of reacting mole ratio calculations
(relating reacting masses and formula
mass)

Using
moles to calculate empirical formula and deduce molecular formula of a compound/molecule
(starting with reacting masses or % composition)

Moles and the molar volume of a gas, Avogadro's Law

Reacting gas volume
ratios, Avogadro's Law
and GayLussac's Law (ratio of gaseous
reactantsproducts)

Molarity, volumes and solution
concentrations (and diagrams of apparatus)

How to
do volumetric titration calculations e.g. acidalkali titrations
(and diagrams of apparatus)

Electrolysis products calculations (negative cathode and positive anode products)

Other calculations
e.g. % purity, % percentage & theoretical yield, volumetric titration
apparatus, dilution of solutions
(and diagrams of apparatus), water of crystallisation, quantity of reactants
required, atom economy

Energy transfers in physical/chemical changes,
exothermic/endothermic reactions

Gas calculations involving PVT relationships,
Boyle's and Charles Laws

Radioactivity & halflife calculations including
dating materials
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