Mass, moles and particles

5 minAS Level2.2.1

AI-drafted, independently checked · not yet teacher-reviewed

By the end of this lesson

By the end you can use molar mass to convert between mass, amount in moles and number of particles, and calculate the mass of a single atom or molecule.

Weighing is counting

You can't count $10^{23}$ atoms one by one. But you can weigh them. The mole turns a balance into a particle counter.

The bridge is the molar mass, $M$ : the mass of one mole of a substance, in $\text{g mol}^{-1}$ . Here's the gift: $M$ is numerically equal to $A_r$ or $M_r$ .

substance	$A_r$ or $M_r$	molar mass, $M$
C	12.0	$12.0\ \text{g mol}^{-1}$
$\text{H}_2\text{O}$	18.0	$18.0\ \text{g mol}^{-1}$
$\text{CaCO}_3$	100.1	$100.1\ \text{g mol}^{-1}$

Every route goes through moles. Never jump straight from grams to particles.

The toolkit

$n = \frac{m}{M} \qquad\qquad N = n \times L$

$m$ = mass in g (convert kg or mg first)
$M$ = molar mass in $\text{g mol}^{-1}$
$n$ = amount in mol; $N$ = number of particles; $L = 6.022 \times 10^{23}\ \text{mol}^{-1}$

Units check: $\text{g} \div \text{g mol}^{-1} = \text{mol}$ . If your units don't cancel to mol, something's upside down.

Quick check

CalculateQuestion 1 · Recall

CalculateQuestion 2 · Recall

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