Concentration of Solutions

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CognitoConcentration calculations

Key facts & methods

Concentration formula

Concentration = amount of solute dissolved in a given volume of solution.
Unit: mol/dm³ (moles per decimetre cubed = moles per litre).
Formula: concentration (mol/dm³) = moles ÷ volume (dm³).
Rearranged: moles = concentration × volume. Volume = moles ÷ concentration.
1 dm³ = 1000 cm³ = 1 litre. To convert cm³ to dm³: divide by 1000.

Worked example

Q: What is the concentration of a solution containing 0.2 mol of NaOH in 500 cm³?
Step 1: Convert volume to dm³: 500 ÷ 1000 = 0.5 dm³.
Step 2: concentration = moles ÷ volume = 0.2 ÷ 0.5 = 0.4 mol/dm³.
Q: How many moles of HCl are in 250 cm³ of 2 mol/dm³ solution?
Step 1: Volume = 250 ÷ 1000 = 0.25 dm³.
Step 2: moles = concentration × volume = 2 × 0.25 = 0.5 mol.

Mass concentration

Concentration can also be expressed as g/dm³.
To convert: multiply mol/dm³ by the molar mass (Mᵣ).
E.g. 2 mol/dm³ NaOH: Mᵣ = 23+16+1 = 40. Concentration = 2 × 40 = 80 g/dm³.
Titration: used to find the concentration of an unknown solution using a known concentration standard.
At the equivalence point: moles of acid = moles of alkali (for 1:1 reactions).

Exam questions — 4 questions · 10 marks · AQA 8462 style

3 marksConcentration calculationAQA 8462 P2 style 🔢 Calculator

(a) Calculate the concentration of a solution containing 0.15 mol of KOH in 300 cm³. (b) How many moles of sulfuric acid are in 150 cm³ of 0.5 mol/dm³ H₂SO₄? (3 marks)

Hint: Always convert cm³ to dm³ first (÷1000). Then apply: concentration = moles/volume or moles = concentration × volume.

+30 XP

3 marksConvert to g/dm³AQA 8462 style 🔢 Calculator

A solution of sodium hydroxide (NaOH) has concentration 0.4 mol/dm³. Calculate the concentration in g/dm³. (Mᵣ: Na=23, O=16, H=1) (2 marks)

Hint: Concentration in g/dm³ = concentration in mol/dm³ × molar mass.

+30 XP

1 markUnit conversionAQA 8462 style

250 cm³ is equal to:

A 0.025 dm³

B 0.25 dm³

C 2.5 dm³

D 250 dm³

Concentration of Solutions

Concentration formula

Worked example

Mass concentration

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