Amount of Substance — The Mole — Edexcel IAL Chemistry Notes

The mole and Avogadro's constant, molar mass, reacting masses, gas volumes and solution concentrations.

The mole is the chemist's counting unit: $6.02 \times 10^{23}$ particles (the Avogadro constant). Mole calculations are tested in every paper.

The key relationships

$n = \frac{m}{M_r} \qquad n = c \times V \qquad n = \frac{V_\text{gas}}{24}\ (\text{dm}^3\text{ at rtp})$

where $n$ = moles, $m$ = mass (g), $M_r$ = molar mass (g mol⁻¹), $c$ = concentration (mol dm⁻³), $V$ = volume of solution (dm³).

Worked example. How many moles in $50$ g of CaCO₃? $M_r = 40 + 12 + 48 = 100$ , so $n = \dfrac{50}{100} = 0.5$ mol.

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The key relationships

n = \frac{m}{M_r} \qquad n = c \times V \qquad n = \frac{V_\text{gas}}{24}\ (\text{dm}^3\text{ at rtp})

where

n

= moles,

m

= mass (g),

M_r

= molar mass (g mol⁻¹),

c

= concentration (mol dm⁻³),

V

= volume of solution (dm³).

Worked example. How many moles in

50

g of CaCO₃?

M_r = 40 + 12 + 48 = 100

, so

n = \dfrac{50}{100} = 0.5

mol.