Concentration of Solutions (AQA GCSE Chemistry): Revision Notes
Concentration of a solution
What is concentration?
The concentration of a solution tells us how much dissolved substance (called the solute) is packed into a certain amount of liquid.
Think of it like making squash - the more concentrated it is, the more squash powder you've dissolved in the water.
Key point: Concentration measures the mass of dissolved substance in 1 dm³ of solution.
Understanding the units
When we make solutions, we need to measure things carefully:
- A solution is made by dissolving a solute in water (the solvent)
- The mass of the solute is measured in grams (g)
- The volume of the solution is measured in dm³ (cubic decimetres)
- Important conversion: 1 dm³ = 1000 cm³ (this is also called one litre)
The concentration formula
To work out concentration, we use this simple formula:
The units for concentration are g/dm³ (grammes per cubic decimetre).
Always ensure your volume is in dm³ before using this formula. If given in cm³, you must convert first by dividing by 1000!
Working through examples
Worked Example 1: Finding mass of dissolved substance
Seawater contains about 35 g/dm³ of sodium chloride. How much sodium chloride is in 500 dm³ of seawater?
Step 1: Use the formula rearranged
Step 2: Put in the numbers
Worked Example 2: Converting units first
Calculate the mass of HCl in 25 cm³ of 4.00 g/dm³ hydrochloric acid.
Step 1: Convert cm³ to dm³
Step 2: Use the formula
Top tip for unit conversion
Unit conversion is essential for accurate calculations. Many students make errors by forgetting to convert cm³ to dm³.
Always remember: when the volume is given in cm³, you must divide by 1000 to convert to dm³ before using the formula.
1000 cm³ = 1 dm³
Key Points to Remember:
- Concentration = mass ÷ volume
- Units are g/dm³
- Always convert cm³ to dm³ by dividing by 1000
- The more solute dissolved, the higher the concentration
- Use the formula triangle: put mass on top, concentration and volume on the bottom