Chapter summary
Chemical Calculations, at a glance
Write formulae and equations, then calculate moles, mass and molar mass.
SEC G2 Science K223 / K225 (2027)
Quick revision
Revisit the essentials, then return to an explanation when you need it.
| Term | Definition to write |
|---|---|
| Relative atomic mass, Ar | The average mass of one atom of an element (taking its isotopes into account), compared with 1/12 of the mass of one atom of carbon-12. |
| Relative molecular mass, Mr | The average mass of one molecule of a substance, compared with 1/12 of the mass of one atom of carbon-12. It is the sum of the relative atomic masses of the atoms in the formula. |
- Formula
Write the correct formula, using ion charges for ionic compounds.
- Equation
Balance atoms with coefficients, never by changing subscripts, and add state symbols.
- Relative mass
Add the Ar values, counting every atom inside brackets.
- Moles
n = m / M, m = n x M or M = m / n, with mass in g and M in g mol-1.
Quick recall
Work out the formula of aluminium oxide.