DOC BROWN'S Science-CHEMISTRY HOMEPAGE KS3 SCIENCE QUIZZES and WORKSHEETS (~US grades 6-8)
GCSE SCIENCE help links GCSE ADDITIONAL SCIENCE help links
KS3 BIOLOGY Quizzes KS3 CHEMISTRY Quizzes & Worksheets KS3 PHYSICS Quizzes
KS4 Science GCSE/IGCSE CHEMISTRY NOTES (~US grades 8-10) KS4 Science GCSE/IGCSE CHEMISTRY QUIZZES and WORKSHEETS (~US grades 8-10) ADVANCED LEVEL CHEMISTRY QUIZZES and WORKSHEETS (~US grades 11-12)
Custom Search

Check out what is available? Study the different examples then try the Quizzes!

% element by Mass - % COMPOSITION OF A COMPOUND

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

Quantitative Chemistry calculations online 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. 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

Spotted any careless error? EMAIL query?comment or request a type of GCSE calculation not covered?

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)

It always seems complicated when stated in this formal way, but the calculations are actually quite easy ..

as long as you can correctly read a formula!

  • 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%,useful arithmetical !
  • 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

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 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

how to do percent by mass calculations Revision KS4 Science revising how to do percent by mass calculations Additional Science Triple Award Science Separate Sciences Courses aid to how to do percent by mass calculations textbook revision GCSE/IGCSE/O level Chemistry how to do percent by mass calculations Information Study Notes for revising for AQA GCSE Science how to do percent by mass calculations, Edexcel GCSE Science/IGCSE Chemistry how to do percent by mass calculations & OCR 21st Century Science, OCR Gateway Science how to do percent by mass calculations  WJEC gcse science chemistry how to do percent by mass calculations CEA/CEA gcse science chemistry O Level Chemistry (revise courses equal to US grade 8, grade 9 grade 10 how to do percent by mass calculations) A level Revision notes for GCE Advanced Subsidiary Level how to do percent by mass calculations AS Advanced Level A2 IB Revising how to do percent by mass calculations AQA GCE Chemistry OCR GCE Chemistry how to do percent by mass calculations Edexcel GCE Chemistry Salters Chemistry how to do percent by mass calculations CIE Chemistry how to do percent by mass calculations, WJEC GCE AS A2 Chemistry how to do percent by mass calculations, CCEA/CEA GCE AS A2 Chemistry revising how to do percent by mass calculations courses for pre-university students (equal to US grade 11 and grade 12 and AP Honours/honors level how to do percent by mass calculations revision guide to how to do percent by mass calculations


top sub-index
Website content copyright Dr Phil Brown 2000-2014 All rights reserved on revision notes, images, puzzles, quizzes, worksheets, x-words etc. * Copying of website material is not permitted * chemhelp@tiscali.co.uk

Alphabetical Index for Science Pages Content A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

**

SITE HELP SEARCH - ENTER SPECIFIC WORDS/FORMULA etc.