Expressing Concentration
Qualitative vs Quantitative
We can say that the solution is dilute (relatively very small quantity of solute)
Or it is concentrated (relatively very large quantity of solute)
But this can lead to confusion, so we need quantitative descriptions.
There are several ways by which we can describe the concentration of the solution quantitatively:
- Mass percentage
- Volume percentage
- Mass by volume percentage
- Parts per million
- Mole fraction
- Molarity
- Molality
Concentration Units
Let's explore the different ways to express concentration quantitatively:
Mass Percentage (w/w)
The mass percentage of a component of a solution is defined as:
Mass % of a component = (Mass of the component in the solution / Total mass of the solution) × 100
Example: If a solution is described by 10% glucose in water by mass, it means that 10g of glucose is dissolved in 90g of water resulting in a 100g solution.
Real-world application: Commercial bleaching solution contains 3.62 mass percentage of sodium hypochlorite in water.
Comparison of Concentration Units
| Unit | Definition | Temperature Dependence | Common Uses |
|---|---|---|---|
| Mass % | (Mass of solute / Total mass) × 100 | Independent | Industrial applications |
| Volume % | (Volume of solute / Total volume) × 100 | Dependent | Liquid solutions |
| PPM | (Parts of component / Total parts) × 106 | Independent | Trace quantities, pollutants |
| Mole Fraction | Moles of component / Total moles | Independent | Vapor pressure calculations |
| Molarity | Moles of solute / Volume in liters | Dependent | Laboratory work |
| Molality | Moles of solute / Mass of solvent in kg | Independent | Colligative properties |