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Molecular Mass

Molecular mass is the sum of the atomic masses of all atoms present in one molecule of a substance.

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Student-friendly explanation

For a molecular substance, read the formula carefully, multiply each element's atomic mass by the number of its atoms, and then add the values. It is expressed in atomic mass units, u.

How to write this in exams

  1. 1

    Start with the exact idea

    Molecular mass is the sum of the atomic masses of all atoms present in one molecule of a substance.

  2. 2

    Then show how to use it

    Write the molecular formula. List each element. Count atoms using subscripts. Multiply by atomic mass. Add all contributions with unit u.

  3. 3

    Add one concrete example

    Molecular mass of H2O = 2 x 1 + 16 = 18 u.

  4. 4

    Avoid this incomplete answer

    For H2SO4, writing 1 + 32 + 16 = 49 u is wrong because it ignores the subscripts 2 and 4.

Definition

Molecular mass is the sum of the atomic masses of all atoms present in one molecule of a substance.

Example

Molecular mass of H2O = 2 x 1 + 16 = 18 u.

Rule to remember

Molecular mass = sum of atomic masses of all atoms in the molecule.

Memory hook

Subscript means multiply.

Examples and method

Worked example

For H2SO4: hydrogen contribution = 2 x 1 = 2, sulphur = 32, oxygen = 4 x 16 = 64. Total molecular mass = 98 u.

Method to apply

Write the molecular formula. List each element. Count atoms using subscripts. Multiply by atomic mass. Add all contributions with unit u.

Diagram support

A formula breakdown table with element, number of atoms, atomic mass, and contribution helps avoid mistakes.

How CBSE asks it

Asked as direct calculation, formula interpretation, or identifying an error in a student's mass calculation.

Avoid common mistakes

Common confusion

Students often forget to multiply the atomic mass by the subscript, especially in formulae like CO2 or H2SO4.

Common wrong answer

For H2SO4, writing 1 + 32 + 16 = 49 u is wrong because it ignores the subscripts 2 and 4.

Exam tip

Underline each subscript and multiply before adding; do not add atomic masses only once.

Quick check

Why is the molecular mass of CO2 equal to 44 u and not 28 u?

CO2 has one carbon atom and two oxygen atoms. Its molecular mass is 12 + 2 x 16 = 44 u, so oxygen's atomic mass must be counted twice.

Answer writing and exam use

1-mark answer

Molecular mass is the sum of the atomic masses of all atoms present in one molecule of a substance.

2-mark answer

Molecular mass is the sum of the atomic masses of all atoms present in one molecule of a substance. Molecular mass = sum of atomic masses of all atoms in the molecule. Molecular mass of H2O = 2 x 1 + 16 = 18 u.

3-mark answer

For a molecular substance, read the formula carefully, multiply each element's atomic mass by the number of its atoms, and then add the values. It is expressed in atomic mass units, u. Molecular mass = sum of atomic masses of all atoms in the molecule. For H2SO4: hydrogen contribution = 2 x 1 = 2, sulphur = 32, oxygen = 4 x 16 = 64. Total molecular mass = 98 u. Asked as direct calculation, formula interpretation, or identifying an error in a student's mass calculation. For H2SO4, writing 1 + 32 + 16 = 49 u is wrong because it ignores the subscripts 2 and 4.
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