Gas volumes and solutions
Gas volumes and solutions
- Equal volumes of any gases (same conditions) hold equal numbers of molecules.
- One mole of gas takes a fixed volume at r.t.p.
- For solutions, concentration links moles to volume.
Practice
Equal volumes of different gases at the same temperature and pressure contain:
This is Avogadro's law: equal volumes (same conditions) hold equal numbers of molecules, whatever the gas.
Volumes of gases
- At the same temperature and pressure, equal volumes contain equal numbers of molecules.
- At r.t.p., one mole of any gas occupies $24.0\ \text{dm}^3$:
$$n = \frac{V}{24.0}\quad(V \text{ in } \text{dm}^3)$$
Practice
How many moles are in 48 dm³ of gas at r.t.p.? (1 mole = 24 dm³)
n = V / 24 = 48 / 24 = 2 mol.
Concentration and titration
- Concentration $c$ is the moles of solute per $\text{dm}^3$ of solution (units $\dfrac{\text{mol}}{\text{dm}^3}$):
$$n = c \times V \qquad (1000\ \text{cm}^3 = 1\ \text{dm}^3)$$
- A titration finds an unknown concentration by reacting it with a solution of known concentration.
Practice
How many moles of solute are in 250 cm³ of a 0.2 mol/dm³ solution? (250 cm³ = 0.25 dm³)
n = c × V = 0.2 × 0.25 = 0.05 mol.
Practice
A titration is used to:
In a titration, a known solution is added until the reaction is complete, revealing the unknown concentration.
Significant figures
- Give answers to a sensible number of significant figures — usually match the data.
- Don't write more digits than the data supports, and don't round too early.
You've got it
Key idea
- equal gas volumes (same T, P) = equal numbers of molecules
- at r.t.p. one mole of gas = $24.0\ \text{dm}^3$, so $n = \dfrac{V}{24.0}$
- solutions: $n = c \times V$ ($c$ in $\dfrac{\text{mol}}{\text{dm}^3}$, $V$ in $\text{dm}^3$; $1000\ \text{cm}^3 = 1\ \text{dm}^3$)
- a titration finds an unknown concentration; quote a sensible number of significant figures