Relative formula mass: Students should be able to calculate the percentage by mass in a compound given the relative formula mass and the relative atomic masses.

The idea

Percentage by mass shows the fraction of a compound's mass contributed by a particular element. First calculate the relative formula mass (Mr) of the compound by adding the relative atomic masses (Ar) of all atoms in its formula. Then find the total mass of the element of interest by multiplying its Ar by the number of its atoms in the formula. Divide that element's total mass by the compound's Mr and multiply by 100. For example, in H₂O, Mr = (2 × 1) + 16 = 18; the percentage by mass of hydrogen is (2 ÷ 18) × 100 = 11.1%. The method is assessed by checking correct Ar values, correct counting of atoms, correct Mr, and correct substitution into the percentage expression.

What earns the marks

• Calculate the relative formula mass of the compound by summing the relative atomic masses of all atoms in the formula.
• Identify the number of atoms of the required element in the formula.
• Calculate the total mass of that element as Ar × number of atoms.
• Divide the element's total mass by the compound's Mr and multiply by 100.
• Give the answer to an appropriate number of significant figures, typically three.

Common mistakes and how to put them right

• Using the relative atomic mass of the element instead of its total mass in the compound.

  Put it right by multiplying Ar by the number of atoms present.

• Dividing by the element's mass rather than the compound's Mr.

  Put it right by using Mr as the denominator.

• Forgetting to multiply by 100.

  Put it right by converting the fraction to a percentage.

Exam tips

• Write out the Mr calculation line by line to avoid arithmetic slips.
• State the formula percentage = (total mass of element ÷ Mr) × 100 before substituting values.
• Check that your final percentage is less than 100 and is reasonable for the formula.