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Formula Unit Mass

Formula unit mass is the sum of the atomic masses of all ions or atoms shown in the formula unit of an ionic compound.

In one line: Ionic lattice uses formula unit, not molecule.

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Understand it

Ionic compounds do not usually exist as separate molecules. Their formula represents the simplest whole-number ratio of ions in the lattice. So for ionic compounds, we calculate formula unit mass instead of molecular mass.

Rule to remember

Formula unit mass = sum of atomic masses of atoms or ions represented in one formula unit of an ionic compound.

See it worked out

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

Worked example

For CaCl2, formula unit mass = 40 + 2 × 35.5 = 111 u.

Method you can reuse

  1. 1Identify the compound as ionic.
  2. 2Read the formula unit.
  3. 3Multiply each atomic mass by its subscript.
  4. 4Add all contributions and write unit u.

Visual tip

A small ionic lattice sketch can show that NaCl represents the simplest ion ratio, not a single molecule.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why is the mass of NaCl called formula unit mass rather than molecular mass?

Think first, then show answer
NaCl is an ionic compound arranged as a lattice of Na+ and Cl- ions, not as separate NaCl molecules. Therefore, the mass calculated from NaCl is called formula unit mass.

Common wrong answer

For CaCl2, writing 75.5 u.

Why it loses marks: It counts only one chlorine atom instead of two.

Don't mix this up

Students call NaCl molecular mass. It is better to write formula unit mass because NaCl is an ionic compound with a lattice structure.

Exam tip

First identify whether the compound is ionic. If it is ionic, use the term formula unit mass and calculate from the formula.

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 a calculation and as a distinction between molecular mass of covalent substances and formula unit mass of ionic substances.
  1. 1

    Enough for 1 mark

    Formula unit mass is the sum of the atomic masses of all ions or atoms shown in the formula unit of an ionic compound.

  2. 2

    Add for 2 marks

    Example: Formula unit mass of NaCl = 23 + 35.5 = 58.5 u.

  3. 3

    Add for 3 marks

    Ionic compounds do not usually exist as separate molecules.

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