Concentration of a solution Revision Notes for AQA GCSE Chemistry Combined Science

Concentration of a solution

What is concentration?

Concentration tells us how much substance is dissolved in a solution. Think of it like making squash - the more concentrated the squash, the stronger the taste because there's more squash dissolved in the same amount of water.

In chemistry, concentration measures how much solute (the dissolved substance) is present in a fixed volume of solution (the mixture of solute and solvent).

Understanding the key terms

When we make a solution, we need to understand these fundamental concepts:

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Key Definitions:

Solution - the final mixture you get
Solute - the substance that dissolves (like sugar or salt)
Solvent - the liquid that does the dissolving (usually water)

Units we use

For concentration calculations, we work with specific units to ensure accuracy:

Mass of solute: measured in grammes (g)
Volume of solution: measured in cubic decimetres (dm³)
Concentration: measured in grammes per cubic decimetre (g/dm³)

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Critical Conversion: 1 dm³ = 1000 cm³ = 1 litre

This conversion is essential for solving concentration problems correctly.

The concentration formula

Here's the fundamental relationship that connects all concentration calculations:

$\text{Concentration} = \frac{\text{Mass of solute}}{\text{Volume of solution}}$

Or written with units:

$\text{Concentration (g/dm³)} = \frac{\text{Mass (g)}}{\text{Volume (dm³)}}$

Converting volumes

Volume conversions are frequently needed in concentration problems:

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Worked Example: Volume Conversion

To change cm³ to dm³: divide by 1000

Step 1: Identify the volume in cm³ Volume = 250 cm³

Step 2: Apply the conversion 250 cm³ ÷ 1000 = 0.25 dm³

Working through examples

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Worked Example 1: Finding concentration

If 75g of solid dissolves to make 3 dm³ of solution, find the concentration.

Solution: Using the formula: $\text{Concentration} = \frac{\text{Mass}}{\text{Volume}}$

Concentration = $\frac{75\text{g}}{3\text{dm}^3} = 25\text{ g/dm}^3$

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Worked Example 2: Converting units first

If 20g of solid dissolves in 250 cm³ of solution, find the concentration.

Step 1: Convert volume to dm³ 250 cm³ = 250 ÷ 1000 = 0.25 dm³

Step 2: Calculate concentration Concentration = $\frac{20\text{g}}{0.25\text{dm}^3} = 80\text{ g/dm}^3$

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Worked Example 3: Finding mass

If seawater has a concentration of 35 g/dm³, how much salt is in 500 dm³?

Step 1: Rearrange the formula Mass = concentration × volume

Step 2: Substitute values Mass = 35 × 500 = 17,500g

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Key Points to Remember:

Concentration shows how much solute is in 1 dm³ of solution
Always use the formula: $\text{concentration} = \frac{\text{mass}}{\text{volume}}$
Convert cm³ to dm³ by dividing by 1000
Units for concentration are g/dm³
You can rearrange the formula to find mass or volume if needed

Concentration of a solution (AQA GCSE Chemistry Combined Science): Revision Notes