Moles in Solutions: Concentration and Titrations

When substances are dissolved in water to form solutions, we need a way to describe how much solute is present in a given volume. This is where concentration comes in — and titrations provide a precise experimental method for determining unknown concentrations.

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Concentration

Concentration can be expressed in two ways at A-Level:

Mol dm⁻³ (moles per cubic decimetre): This tells you how many moles of solute are dissolved in 1 dm³ (1000 cm³) of solution.

g dm⁻³ (grams per cubic decimetre): This tells you the mass of solute dissolved in 1 dm³ of solution.

The relationship between the two is:

concentration (g dm⁻³) = concentration (mol dm⁻³) × molar mass (g mol⁻¹)

The Key Equation

n = c × V

Where:

• n = number of moles (mol)
• c = concentration (mol dm⁻³)
• V = volume (dm³)

Important: Volume must be in dm³, not cm³. To convert: V (dm³) = V (cm³) / 1000

Worked Example 1

Calculate the number of moles of NaOH in 25.0 cm³ of 0.100 mol dm⁻³ sodium hydroxide solution.

Step 1: Convert volume to dm³. V = 25.0 / 1000 = 0.0250 dm³