Relative formula mass: The relative formula mass (M_r) of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.

The idea

Relative formula mass compares the mass of one formula unit of a compound with one-twelfth of the mass of a carbon-12 atom, so it has no units. To calculate it, use the relative atomic masses of the elements, usually given in the periodic table or the question, and add them in the proportions shown by the formula. For water, H₂O, the calculation is (2 × 1) + 16 = 18. For calcium carbonate, CaCO₃, it is 40 + 12 + (3 × 16) = 100. Subscripts multiply the relative atomic mass of the atom or group they follow, and brackets are treated in the same way. Balancing numbers (coefficients) in front of a formula are ignored when calculating relative formula mass.

What earns the marks

• Relative formula mass is the sum of the relative atomic masses of all atoms shown in the formula of the compound.
• Each subscript in the formula multiplies the relative atomic mass of the atom or group immediately before it.
• Relative formula mass is calculated from the formula alone; balancing numbers (coefficients) in front of a formula are ignored.
• The value of M_r is a ratio and therefore has no units.
• Calculations should use the relative atomic masses supplied in the question or taken from the periodic table, showing each element's contribution.
• For formulae containing brackets, the subscript after the bracket multiplies every atom inside the bracket.

Common mistakes and how to put them right

• Adding subscripts as if they were masses, for example treating H₂O as 1 + 2 + 16.

  Put it right by multiplying each relative atomic mass by its subscript.

• Ignoring a subscript when several atoms of one element are present, for example calculating CO₂ as 12 + 16.

  Put it right by including (2 × 16) for the two oxygen atoms.

• Including balancing coefficients in the calculation, for example calculating the M_r of 2H₂O as 36.

  Put it right by ignoring the front multiplier and calculating the M_r of H₂O as 18.

Exam tips

• Write out each element's contribution separately before adding, so a marker can follow the method and any slip is easy to find.
• Do not use balancing numbers (coefficients) in front of a formula when calculating its relative formula mass (M_r).