Doc Brown's Chemistry - Chemical Calculations

On-line 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)

The shorthand M_r can be used for any element or compound. Whereas relative atomic mass (1. above) 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, 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.
  • 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
  • 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
  • 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
  • M_r = 40 + 2 x (16+1) = 74
• Example 2.6:

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