Why composition matters
Two solutions can hold the same solute in the same solvent and still not be the same solution. What separates them is how much solute there is, and that is what composition means.
Activity 3.2.1 — method
Put 50 ml of water into each of two beakers. Add 0.2 g of potassium permanganate to one and 0.4 g of potassium permanganate to the other. Stir both well with a glass rod and write down what you see.
The observation
The solution with 0.2 g in it is a pale purple. The one with 0.4 g is a deeper purple than that.
Reading the colour
The two beakers took equal volumes of water, so the amount of solvent is the same. The masses of potassium permanganate are not. The deeper purple solution holds more solute particles in a unit volume — and that is why the composition of these two solutions is different from each other.
Where an exact composition is not optional
Getting this right is not a laboratory nicety.
- Weedicides and pesticides mixed with water for spraying must be made up to the right composition
- Mixtures made with medicines must use the specified composition
- Laboratory work very often needs solutions of an exact composition
Five ways to state it
The composition of a mixture can be expressed in a number of ways, and the rest of this skill and the next take them one at a time.
| Way of stating it | What it compares |
|---|---|
| Mass fraction (m/m) | The mass of one component against the total mass |
| Volume fraction (v/v) | The volume of one component against the total volume |
| Mole fraction | The moles of one component against the total moles |
| Mass / volume (m/v) | The mass of solute in a unit volume of solution |
| Moles / volume (n/v) | The moles of solute in a unit volume — this one is called concentration |