Doc Brown's Chemistry - Chemical Calculations

Online 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?

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
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