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

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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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 quite correct to talk about the molecular mass of an ionic compound when it doesn't consist of molecules!

The shorthand M_r can be used for the formula of any element or compound. Whereas relative atomic mass (section 1. Relative Atomic Mass) only applies to a single atom but anything with at least two atoms requires the term relative formula mass or relative molecular mass.

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

Examples of formula/molecular mass calculations:

• Recap: Molecular/formula mass = total of all the atomic masses of all the atoms in the molecule/compound.
• 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 M_r, are H₂ = 2 x 1 = 2, Cl₂ = 2 x 35.5 = 71 respectively.
• Example 2.2: the element phosphorus consists of P₄ molecules.
  • RMM or M_r of phosphorus = 4 x its atomic mass = 4 x 31 = 124
• Example 2.3: The compound water H₂O
  • relative atomic masses are H=1 and O=16
  • RMM or M_r = (1x2) + 16 = 18 (molecular mass of water)
• Example 2.4: The compound sulphuric acid H₂SO₄
  • relative atomic masses are H=1, S=32 and O=16
  • RMM or M_r = (1x2) + 32 + (4x16) = 98 (molecular mass of sulphuric acid)
• Example 2.5: The compound calcium hydroxide Ca(OH)₂ (ionic)
  • relative atomic masses are Ca=40, H=1 and O=16
  • RMM or M_r = 40 + 2 x (16+1) = 74
• Example 2.6: The ionic compound aluminium oxide (Al³⁺)₂(O^2-)₃ or just plain Al₂O₃
  • relative atomic masses are Al = 27 and O = 16
  • so the formula mass RFM or M_r = (2 x 27) + (2 x 16) = 102
• Example 2.7: calcium phosphate is also ionic but a more tricky formula to work out!
  • (Ca²⁺)₃(PO₄^3-)₂ or Ca₃(PO₄)₃
  • atomic masses: Ca = 40, P = 31, O =16
  • RFM or M_r = (3 x 40) + 3 x {31 + (4 x 16)} = (120) + (3 x 95) = 405
• -

Self-assessment Quizzes

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 (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

   • Reacting masses, concentration of solution and volumetric titration calculations (NOT using moles)

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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