TCE Physical Sciences Practice Questions

Relative atomic mass is the weighted average of the isotopic masses, so multiply each mass number by its percentage abundance and divide by 100. A(r) = [(24 x 78.99) + (25 x 10.00) + (26 x 11.01)] / 100 A(r) = (1895.76 + 250.0 + 286.26) / 100 A(r) = 2432.02 / 100 A(r) = 24.32 Relative atomic mass has no unit, because it is a ratio of the weighted average mass per atom to 1/12 the mass of an atom of carbon-12 — do not write g mol^-1 here. The answer sits very close to magnesium-24 because that isotope makes up almost four-fifths of the sample, which is a quick sanity check that the weighting has been applied the right way round. The value on the periodic table is 24.31; the small difference arises because this calculation used whole-number mass numbers rather than the exact isotopic masses.