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medium difficulty5 min

Molecular Mass

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

In one line: Subscript means multiply.

Practice This Concept

Understand it

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.

Example

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

Rule to remember

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

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

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

Method you can reuse

  1. 1Write the molecular formula.
  2. 2List each element.
  3. 3Count atoms using subscripts.
  4. 4Multiply by atomic mass.
  5. 5Add all contributions with unit u.

Visual tip

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

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

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

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

Common wrong answer

For H2SO4, writing 1 + 32 + 16 = 49 u.

Why it loses marks: It ignores the subscripts 2 and 4.

Don't mix this up

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

Exam tip

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

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Asked as direct calculation, formula interpretation, or identifying an error in a student's mass calculation.
  1. 1

    Enough for 1 mark

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

  2. 2

    Add for 2 marks

    Example: Molecular mass of H2O = 2 × 1 + 16 = 18 u.

  3. 3

    Add for 3 marks

    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.

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