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

2. Calculating relative formula mass or relative molecular mass RFM or Mr

Help for problem solving in doing relative formula mass calculations using atomic masses. Practice revision questions on calculating relative atomic mass from a chemical formula and atomic masses. This page describes, and explains, with worked out examples, the method of how to calculate the relative formula mass of a compound (ionic or covalent) or the relative molecular mass of an element or a covalent compound.

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study examples carefully2. How to calculate relative formula mass or relative molecular mass RFM/RMM or Mrstudy examples carefully

How do I calculate relative molecular mass? RMM

How to calculate relative formula mass? RFM

Is there any difference between RMM and RFM?

Does it matter whether the compound is ionic or covalent?

If all the individual atomic masses of all the atoms in a formula are added together you have calculated the relative formula mass (for ionic compounds e.g. NaCl = 58.5) or molecular mass (for covalent elements e.g. N2 = 28 or compounds e.g. C6H12O6 = 180).

To be honest, the term relative formula mass can be used with any compound whether it be ionic or covalent - it just seems NOT correct to talk about the molecular mass of an ionic compound when it doesn't consist of molecules, but is that one for the purists!

The shorthand Mr can be used for the formula of any element or compound and to repeat, 'it doesn't matter whether a compound is ionic or covalent'.

Mr = Relative formula mass = relative molecular mass = the sum of all the atomic masses for all the atoms in a given formula

NOTE: You cannot successfully calculate formula/molecular masses if you cannot read a formula correctly!

Whereas relative atomic mass (section 1. Relative Atomic Mass) only applies to a single atom, anything with at least two atoms in the formula requires the term relative formula mass or relative molecular mass to be used.

The most common error is to use atomic/proton numbers instead of atomic masses, unfortunately, except for hydrogen, they are different!

Examples of relative formula/molecular mass calculations:

How to calculate relative molecular mass = How to calculate relative formula mass

Recap: Molecular/formula mass = total of all the atomic masses of all the atoms in the molecule/compound.

  • Molecular/formula mass calculation Example 2.1
    • The diatomic molecules of the elements hydrogen H2 and chlorine Cl2
    • relative atomic masses, Ar: H = 1, Cl = 35.5
    • Formula masses, RMM or Mr
      • are for hydrogen H2 = 2 x 1 = 2
      • and for chlorine Cl2 = 2 x 35.5 = 71 respectively.
  • Molecular/formula mass calculation Example 2.2
    • The element phosphorus consists of P4 molecules.
    • RMM or Mr of phosphorus = 4 x its atomic mass = 4 x 31 = 124
  • Molecular/formula mass calculation Example 2.3: The compound water H2O
    • relative atomic masses are H=1 and O=16
    • RMM or Mr = (1x2) + 16 = 18 (molecular mass of water)
  • Molecular/formula mass calculation Example 2.4
    • The compound sulphuric acid H2SO4
    • relative atomic masses are H=1, S=32 and O=16
    • RMM or Mr = (1x2) + 32 + (4x16) = 98 (molecular mass of sulphuric acid)
  • Molecular/formula mass calculation Example 2.5
    • The compound calcium hydroxide Ca(OH)2 (ionic)
    • relative atomic masses are Ca=40, H=1 and O=16
    • RMM or Mr = 40 + 2 x (16+1) = 74
  • Molecular/formula mass calculation Example 2.6
    • The ionic compound aluminium oxide (Al3+)2(O2-)3 or just the plain formula Al2O3,
    • but it makes no difference to the calculation of relative formula mass or relative molecular mass.
    • relative atomic masses are Al = 27 and O = 16
    • so the formula mass RFM or Mr = (2 x 27) + (2 x 16) = 102
  • Molecular/formula mass calculation Example 2.7
    • Calcium phosphate is also ionic but a more tricky formula to work out!
    • (Ca2+)3(PO43-)2 or Ca3(PO4)3, but it makes no difference to the calculation of relative formula mass or relative molecular mass.
    • atomic masses: Ca = 40, P = 31, O =16
    • RFM or Mr = (3 x 40) + 3 x {31 + (4 x 16)} = (120) + (3 x 95) = 405
  • Molecular/formula mass calculation Example 2.7
    • Glucose C6H12O6
    • atomic masses: C = 12, O= 16, H = 1
    • Molecular mass of glucose Mr(C6H12O6) = (6 x 12) + (12 x 1) + (6 x 16) = 180

top sub-indexSelf-assessment Quizzes

[rfm] 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 (this page)

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

  4. Composition by percentage mass of elements in a compound

  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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