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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

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.

On-line Quantitative Chemistry Calculations

Online practice exam chemistry CALCULATIONS and solved problems for KS4 Science GCSE/IGCSE CHEMISTRY and basic starter chemical calculations for A level AS/A2/IB courses * EMAIL query?comment or request for type of GCSE calculation?

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 as follows:

(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 element 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)

  • 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 = 64 x 100 / 160 = 40% copper by mass in the compound
  • 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 = 192 x 100 / 342 = 56.1% oxygen by mass in aluminium sulphate
  • Example 4.3: 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 = 126 x 100 / 246 = 51.2 %
    • Note: The determination and calculation of the formula of a hydrated salt like MgSO4.7H2O is covered in Calculations section 14.4.
  • Example 4.4:

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

OTHER CALCULATION PAGES

  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 volumetric titration calculations e.g. acid-alkali titrations (and diagrams of apparatus)

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

  14. 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

  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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