What Is Concentration?

Concentration describes how much solute is dissolved in a given volume of solution.

In simple terms, concentration tells us how strong or dilute a solution is.

For example:

• A solution with a large amount of solute is concentrated
• A solution with a small amount of solute is dilute

Understanding concentration is important for many chemistry calculations and laboratory experiments.

Solute, Solvent and Solution

Before understanding concentration, it is important to know these key terms.

Solute
The substance that is dissolved.

Example: Salt in salt water.

Solvent
The liquid that dissolves the solute.

Example: Water.

Solution
The mixture formed when a solute dissolves in a solvent.

Example: Salt water.

Units of Concentration

In O Level Chemistry, concentration is usually expressed in two main units:

• grams per cubic decimetre (g/dm³)
• moles per cubic decimetre (mol/dm³)

These units help chemists measure the amount of substance in a solution.

Concentration in g/dm³

Concentration in g/dm³ tells us how many grams of solute are dissolved in 1 dm³ of solution.

The formula is:

Concentration (g/dm³) = Mass of solute (g) ÷ Volume of solution (dm³)

Example Calculation (g/dm³)

10 g of salt is dissolved in 2 dm³ of water.

Concentration = 10 ÷ 2

Concentration = 5 g/dm³

This means each cubic decimetre of solution contains 5 grams of salt.

Concentration in mol/dm³

Concentration can also be expressed in moles per cubic decimetre (mol/dm³).

This unit tells us the number of moles of solute in one cubic decimetre of solution.

The formula is:

Concentration (mol/dm³) = Moles of solute ÷ Volume of solution (dm³)

Example Calculation (mol/dm³)

1 mole of sodium chloride is dissolved in 1 dm³ of water.

Concentration = 1 ÷ 1

Concentration = 1 mol/dm³

Converting cm³ to dm³

Sometimes volume is given in cm³, but concentration formulas require dm³.

Conversion rule:

1 dm³ = 1000 cm³

To convert cm³ to dm³:

Divide by 1000

Example:

500 cm³ = 0.5 dm³

Importance of Concentration in Chemistry

Concentration calculations are important for:

• Preparing chemical solutions in laboratories
• Understanding reaction quantities
• Performing titration experiments
• Studying industrial chemical processes

Many chemical reactions depend on the concentration of reactants.

Exam Tip (5070)

Students are commonly asked to:

• Calculate concentration in g/dm³
• Calculate concentration in mol/dm³
• Convert cm³ to dm³

Example exam question:

Calculate the concentration of a solution if 20 g of solute is dissolved in 4 dm³ of water.

Answer:

Concentration = 20 ÷ 4
= 5 g/dm³

Practice Question

2 moles of sodium hydroxide are dissolved in 0.5 dm³ of solution.

Calculate the concentration.

Answer

Concentration = Moles ÷ Volume

= 2 ÷ 0.5

= 4 mol/dm³

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