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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.

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Student-friendly explanation

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.

How to write this in exams

  1. 1

    Start with the exact idea

    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

    Then show how to use it

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

  3. 3

    Add one concrete example

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

  4. 4

    Avoid this incomplete answer

    For CaCl2, writing 75.5 u is wrong because it counts only one chlorine atom instead of two.

Definition

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

Example

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

Rule to remember

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

Memory hook

Ionic lattice uses formula unit, not molecule.

Examples and method

Worked example

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

Method to apply

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

Diagram support

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

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.

Avoid common mistakes

Common confusion

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

Common wrong answer

For CaCl2, writing 75.5 u is wrong because it counts only one chlorine atom instead of two.

Exam tip

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

Quick check

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

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.

Answer writing and exam use

1-mark answer

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-mark answer

Formula unit mass is the sum of the atomic masses of all ions or atoms shown in the formula unit of an ionic compound. Formula unit mass = sum of atomic masses of atoms or ions represented in one formula unit of an ionic compound. Formula unit mass of NaCl = 23 + 35.5 = 58.5 u.

3-mark answer

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. Formula unit mass = sum of atomic masses of atoms or ions represented in one formula unit of an ionic compound. For CaCl2, formula unit mass = 40 + 2 x 35.5 = 111 u. Asked as a calculation and as a distinction between molecular mass of covalent substances and formula unit mass of ionic substances. For CaCl2, writing 75.5 u is wrong because it counts only one chlorine atom instead of two.
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