Relative masses of atoms and molecules

AS Level Chemistry · Atoms, molecules and stoichiometry. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Atoms are far too light to weigh in grams, so their masses are compared with a standard. The standard is the unified atomic mass unit, u, which is one twelfth of the mass of one atom of carbon-12. One carbon-12 atom has a mass of exactly 12 u.

A relative mass is a ratio, so it has no unit. Relative atomic mass, Ar, is a weighted mean because most elements are a mixture of isotopes.

For a substance made of molecules we add the Ar values to get the relative molecular mass, Mr. For an ionic compound, which has no molecules, the same sum is called the relative formula mass.

Rules to remember

Common mistake

Students forget to multiply every atom inside a bracket, or leave out the water in a hydrated salt. Count the atoms of each element before adding.

Worked example

What is the relative formula mass of ammonium sulfate, (NH4)2SO4? (Ar: H 1.0, N 14.0, O 16.0, S 32.1)

  1. Count atoms: N = 2, H = 8, S = 1, O = 4.
  2. Add: (2 × 14.0) + (8 × 1.0) + 32.1 + (4 × 16.0).
  3. = 28.0 + 8.0 + 32.1 + 64.0 = 132.1.

Answer: 132.1 (no unit).

Practice questions

Try each one, then open the answer.

1. The unified atomic mass unit is defined as

  1. A
    one twelfth of the mass of one atom of carbon-12
  2. B
    the mass of one proton
  3. C
    the mass of one atom of hydrogen-1
  4. D
    one twelfth of the mass of one mole of carbon-12
Show answer

Answer: A. By definition 1 u = 1/12 of the mass of a single 12C atom. It is close to, but not exactly, the mass of a proton or a hydrogen atom.

2. Which is the correct definition of relative isotopic mass?

  1. A
    the mass of an atom of an isotope relative to one twelfth of the mass of an atom of carbon-12
  2. B
    the weighted mean mass of the isotopes of an element relative to carbon-12
  3. C
    the number of protons plus neutrons in an atom of an isotope
  4. D
    the mass of one mole of atoms of an isotope
Show answer

Answer: A. Relative isotopic mass refers to a single isotope compared with 1/12 of a carbon-12 atom. A weighted mean describes relative atomic mass, and the nucleon number is a whole-number count, not a relative mass.

3. Relative atomic mass, Ar, is defined as

  1. A
    the weighted mean mass of an atom of an element relative to one twelfth of the mass of an atom of carbon-12
  2. B
    the mass of one atom of an element in grams
  3. C
    the mass of the most abundant isotope relative to carbon-12
  4. D
    the number of protons and neutrons in the nucleus
Show answer

Answer: A. Ar is a weighted mean that takes account of the abundance of each isotope, which is why values such as 35.5 for chlorine are not whole numbers.

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