Calculate the relative molecular mass of water, H_2O (H = 1, O = 16). [1 mark]

Calculate the number of moles in 80\ g of sodium hydroxide, NaOH (M_r = 40). [2 marks]

Calculate the number of atoms in 0.25\ mol of helium (Avogadro constant 6 × 10^23). [2 marks]

SingaporeChemistrySyllabus dot point

What is a mole, and how does it link the mass of a substance to the number of particles it contains?

Define relative atomic and molecular mass, define the mole and the Avogadro constant, and interconvert mass, moles and number of particles

A focused answer to the N(A) Chemistry outcome on the mole. Relative atomic and molecular mass, the mole and the Avogadro constant, and converting between mass, moles and number of particles using simple numbers.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-06

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
The answer
Examples in context
Try this

What this dot point is asking

The syllabus wants you to define relative atomic mass and relative molecular mass, to define the mole and the Avogadro constant, and to convert confidently between mass, moles, and number of particles. The mole is the chemist's counting unit: it links a mass you can weigh to a number of particles you cannot see. At N(A) level the numbers are kept simple, so the focus is on choosing the right conversion.

The answer

Relative atomic mass and relative molecular mass

The relative atomic mass ( $A_r$ ) of an element compares the mass of its atoms to a standard. You read it from the Periodic Table, for example $A_r$ of carbon is $12$ and of hydrogen is $1$ .

The relative molecular mass ( $M_r$ , also called relative formula mass) is found by adding up the relative atomic masses of all the atoms in the formula. For water, $\text{H}_2\text{O}$ :

M_r = (2 \times 1) + 16 = 18

The mole and the Avogadro constant

A mole is the amount of a substance that contains a fixed huge number of particles. That number is the Avogadro constant, about $6 \times 10^{23}$ per mole. One mole of any substance has a mass in grams equal to its $A_r$ or $M_r$ . So one mole of carbon weighs $12\ \text{g}$ and one mole of water weighs $18\ \text{g}$ .

Converting between mass and moles

The key relationship is:

\text{moles} = \frac{\text{mass}}{M_r}

You can rearrange it to find any one of the three quantities:

\text{mass} = \text{moles} \times M_r, \qquad M_r = \frac{\text{mass}}{\text{moles}}

Converting between moles and particles

To go from moles to the number of particles (atoms, molecules, or ions), multiply by the Avogadro constant:

\text{number of particles} = \text{moles} \times 6 \times 10^{23}

Converting mass to moles and then to molecules

A sample of methane, $\text{CH}_4$ , has a mass of $32\ \text{g}$ . Find the number of moles and the number of molecules. (Use H = 1, C = 12, and Avogadro constant $6 \times 10^{23}$ .)

Step 1: Find the relative molecular mass

Add the relative atomic masses in $\text{CH}_4$ :

M_r = 12 + (4 \times 1) = 16

Step 2: Convert mass to moles

Use moles = mass divided by $M_r$ :

n = \frac{32}{16} = 2\ \text{mol}

Step 3: Convert moles to molecules

Multiply the moles by the Avogadro constant:

\text{molecules} = 2 \times 6 \times 10^{23} = 1.2 \times 10^{24}

Step 4: State the answer

The $32\ \text{g}$ sample is $2$ moles, which is $1.2 \times 10^{24}$ molecules of methane. The mass led to moles, and moles led to the particle count.

Examples in context

Example 1. Counting atoms by weighing. A chemist cannot count out $6 \times 10^{23}$ atoms one by one, but can weigh $12\ \text{g}$ of carbon and know it contains exactly one mole. This is why the mole is so useful: it turns an impossible counting job into a simple weighing on a balance.

Example 2. Comparing amounts fairly. Equal masses of two substances do not contain equal numbers of particles, because their formula masses differ. Converting each to moles puts them on the same footing, which is why every reacting-mass calculation starts by turning masses into moles.

Try this

Q1. Calculate the relative molecular mass of water, $\text{H}_2\text{O}$ (H = 1, O = 16). [1 mark]

Cue. $M_r = (2 \times 1) + 16 = 18$ .

Q2. Calculate the number of moles in $80\ \text{g}$ of sodium hydroxide, $\text{NaOH}$ ( $M_r = 40$ ). [2 marks]

Cue. Moles = mass divided by $M_r = 80 \div 40 = 2\ \text{mol}$ .

Q3. Calculate the number of atoms in $0.25\ \text{mol}$ of helium (Avogadro constant $6 \times 10^{23}$ ). [2 marks]

Cue. Atoms = moles multiplied by $6 \times 10^{23} = 0.25 \times 6 \times 10^{23} = 1.5 \times 10^{23}$ atoms.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original4 marksUse these relative atomic masses: H = 1, C = 12, O = 16. (a) Work out the relative molecular mass of carbon dioxide,

\text{CO}_2

. (b) Calculate the number of moles in

88\ \text{g}

of carbon dioxide.

Show worked answer →

(a) Relative molecular mass of $\text{CO}_2$ = $12 + (2 \times 16) = 12 + 32 = 44$ .

(b) Number of moles:

$n = \dfrac{\text{mass}}{M_r} = \dfrac{88}{44} = 2\ \text{mol}$ .

What markers reward: adding the relative atomic masses to get $44$ , using moles = mass divided by $M_r$ , and the answer of $2$ mol.

Original3 marksThe Avogadro constant is

6 \times 10^{23}

per mole. (a) State how many particles are in one mole. (b) Calculate the number of molecules in

0.5\ \text{mol}

of water.

Show worked answer →

(a) One mole contains $6 \times 10^{23}$ particles (the Avogadro constant).

(b) Number of molecules = moles $\times$ Avogadro constant:

$0.5 \times 6 \times 10^{23} = 3 \times 10^{23}$ molecules.

What markers reward: stating $6 \times 10^{23}$ particles per mole, multiplying moles by the Avogadro constant, and the answer $3 \times 10^{23}$ .