GCSE Combined Science (AQA)

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Percentage by Mass Calculations

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Quantitative chemistry Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations

Percentage by Mass Calculations

6:14 Conservation and Changes of Mass in Chemical Reactions
Spec 4.3.1.2
Previous Relative Formula Mass Next Mole Calculations
  • Calculate the percentage of total mass of a compound made up of a single element.
  • These are called percentage by mass calculations.
  • Example: Sodium chloride (table salt) has sodium forming 39% of the mass and chlorine 61%.
  • Formula: Percentage by mass = (AR of element * number of atoms) / MR of compound * 100.
  • Example calculation with lithium fluoride:
  • AR of lithium is 7.
  • MR of lithium fluoride is 26 (7 for lithium + 19 for fluorine).
  • Percentage by mass of lithium = (7 * 1) / 26 * 100 = 26.92%.
  • More complex example: Rubidium in rubidium sulfate (RB���SO���).
  • AR of rubidium is 85, with 2 atoms in the compound.
  • MR of rubidium sulfate is 266.
  • Percentage by mass of rubidium = (85 * 2) / 266 * 100 = 63.91% (rounded to two decimal places).

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