Concentration of solutions — AQA GCSE Combined Science

Concentration of a solution measures the mass of solute dissolved in a given volume of solvent, most commonly calculated using the equation concentration = mass / volume in g/dm3. Mastering this topic requires converting volumes from cm3 to dm3 by dividing by 1000 and understanding how altering solute mass or solvent volume affects concentration.

Concentration of solutions is a fundamental quantitative chemistry topic in AQA GCSE Combined Science. It describes the amount of a chemical substance (solute) dissolved in a defined volume of liquid (solvent), quantified in grams per cubic decimetre (g/dm3).

Understanding concentration enables students to analyze chemical formulations, interpret titration and neutralisation experiments, and link quantitative calculations to real-world industrial and biological processes. It bridges basic laboratory measurement skills with mathematical chemical analysis.

• →Concentration formula: concentration (g/dm3) = mass of solute (g) / volume of solution (dm3).
• →Unit conversion: 1 dm3 = 1000 cm3 = 1 litre; always divide cm3 by 1000 to convert to dm3.
• →Proportionality: Concentration increases if more solute is added to the same volume, or if solvent volume decreases by evaporation while solute mass remains constant.
• →Rearranging equations: Mass (g) = concentration (g/dm3) x volume (dm3), and volume (dm3) = mass (g) / concentration (g/dm3).

• State the relationship concentration = mass of solute ÷ volume of solution and rearrange it correctly to mass = concentration × volume.
• Convert a volume given in cm³ into dm³ by dividing by 1000 before substituting, since concentration is normally quoted in g/dm³.
• Substitute the known concentration and converted volume into mass = concentration × volume and evaluate accurately.
• Give the final mass with the correct unit, typically g, and check that the magnitude is reasonable for the volume and concentration involved.
• Interpret the result in context, for example stating the mass of solute present in the stated volume of that particular solution.

• 💡Write down the equation mass = concentration × volume before substituting any numbers, so the examiner can follow your method.
• 💡Convert every volume to dm³ in a clearly shown step, since unit errors are the most common source of lost marks in this topic.
• 💡Estimate the answer first, for example 40 g/dm³ × 0.25 dm³ is about 10 g, then compare your calculated value with the estimate to catch slips.
• 💡Write out the rearranged formula before substituting numbers to secure method marks even if a calculator entry error occurs.
• 💡Check your units at the end of the calculation; ensure you have written g/dm3 and not g/cm3 unless specifically instructed.
• 💡Remember that 1 cm3 is equivalent to 1 ml and 1 dm3 is equivalent to 1 litre, which helps in contextual questions.

• Multiplying concentration by a volume left in cm³ instead of converting to dm³; correct this by dividing cm³ by 1000 first, so 500 cm³ becomes 0.500 dm³.
• Using mass = volume ÷ concentration instead of mass = concentration × volume; correct this by rearranging concentration = mass ÷ volume carefully before substituting.
• Forgetting the unit on the answer or writing an impossible unit such as g/dm³ for a mass; correct this by checking that the unit matches the quantity calculated, which is grams here.
• Thinking 1 dm3 equals 100 cm3 instead of 1000 cm3, which causes answers to be incorrect by an order of magnitude.
• Believing adding more water makes a solution stronger, confusing total volume with concentration.
• Multiplying cm3 by 1000 instead of dividing when attempting to convert volume to dm3.

1. 1Day 1-2: Memorise the core formula triangle relating mass, volume, and concentration, and practice converting cm3 to dm3.
2. 2Day 3-4: Work through direct calculation questions finding concentration when mass and volume are provided.
3. 3Day 5-6: Practice rearranging the formula to solve for unknown mass and unknown volume under timed conditions.
4. 4Day 7: Tackle multi-step past paper exam questions involving solution dilution, evaporation, and comparison of concentrations.

• 📋Direct calculations: Standard 2-mark or 3-mark questions asking you to calculate concentration in g/dm3 given mass in grams and volume in cm3.
• 📋Rearranged calculations: Questions asking for the mass of solute present in a specific volume of a known concentration.
• 📋Comparative explanation questions: Describing and explaining how adding water or evaporating solvent alters the concentration of a given mixture.