Gas volumes and concentration
Volumes of gases
- At room temperature and pressure (r.t.p.), one mole of any gas takes up the same volume: the molar gas volume, $24\ \text{dm}^3$.
$$\text{volume (dm}^3) = \text{amount (mol)} \times 24$$
Practice
What volume (in dm³) does 2 mol of gas occupy at r.t.p.? (molar gas volume = 24 dm³)
volume = amount × 24 = 2 × 24 = 48 dm³.
Concentration of solutions
- Concentration can be given in $\text{g/dm}^3$ or $\text{mol/dm}^3$.
$$\text{concentration (mol/dm}^3) = \frac{\text{amount of solute (mol)}}{\text{volume (dm}^3)}$$
- Convert volume first: $1\ \text{dm}^3 = 1000\ \text{cm}^3$, so divide a volume in $\text{cm}^3$ by 1000.
Practice
What is the concentration (mol/dm³) of a solution with 0.5 mol of solute in 2 dm³?
concentration = moles ÷ volume = 0.5 ÷ 2 = 0.25 mol/dm³.
Practice
How many cm³ are in 1 dm³?
1 dm³ = 1000 cm³, so divide a cm³ volume by 1000 to get dm³.
You've got it
Key idea
- at r.t.p., 1 mol of any gas = 24 dm³; volume = mol × 24
- concentration (mol/dm³) = moles ÷ volume
- remember 1 dm³ = 1000 cm³