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Doc Brown's Chemistry - GCSE/IGCSE/GCE (basic A level) O Level  Online Chemical Calculations

study examples carefully4. Calculating the % composition by mass of elements in compoundsstudy examples carefully

Help for problem solving in doing percent of mass of elements in a given compound formula calculations. Practice revision questions on % composition of an element in compound, using experiment data, making predictions. 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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study examples carefully4. Method of calculating the % percentage by mass of the elements in a compoundsstudy examples carefully

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. (c) doc bclick me!2. Calculating relative formula/molecular mass (Mr) 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)

Percentage of an element Z in a compound = 100  x atomic mass of Z x number of atoms of Z in the compound formula / formula mass of the compound containing atoms of Z

% by mass of Z = 100 x Ar(Z) x atoms of Z / Mr(compound)

  • Calculation of % composition Example 4.1
    • Calculate the % of copper in copper sulphate, CuSO4
    • 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 = 100 x 64 / 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, Al2(SO4)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 = 100 x 192 / 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 MgSO4.7H2O
    • 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 = 100 x 126 / 246 = 51.2 % H2O
    • Note: The determination and calculation of the formula of a hydrated salt like MgSO4.7H2O 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 Na2SO4, atomic masses: Na = 23, S = 32, O = 16
    • Formula mass Na2SO4 = (2 x 23) + 32 + (4 x 16) = 142
    • Formula mass of sulfate ion SO42- (or just SO4 will do for the calculation) = 32 + (4 x 16) = 96
    • Therefore % sulfate ion in sodium sulfate = (96/142) x 100 = 67.6% SO4

top sub-indexSelf-assessment Quizzes F = easier-foundation, H = harder-higher (but does include F questions)

[pbm] type in answer click me for QUIZ! for F and H  or  multiple choice click me for QUIZ! for F and H


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

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

  3. Law of Conservation of Mass and simple reacting mass calculations

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

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

  6. 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

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

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

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

  10. Reacting gas volume ratios, Avogadro's Law and Gay-Lussac's Law (ratio of gaseous reactants-products)

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

  12. How to do acid-alkali titration calculations, diagrams of apparatus, details of procedures

  13. Electrolysis products calculations (negative cathode and positive anode products)

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

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

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

  17. Radioactivity & half-life calculations including dating materials

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