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Concentration in g/dm³ = mass of solute (g) ÷ volume of solution (dm³), and to convert cm³ to dm³ divide by 1000. Free notes and 10 questions answered.
Understanding Solution Concentration is part of Use of amount of substance in relation to masses of pure substances in GCSE AQA Higher Chemistry.
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Concentration in g/dm³ = mass of solute (g) ÷ volume of solution (dm³), and to convert cm³ to dm³ divide by 1000. Free notes and 10 questions answered.
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Use this lesson to stay inside Use of amount of substance in relation to masses of pure substances while connecting the detail back to the wider Topic 3: Quantitative chemistry topic.
Read the core explanations from Understanding Solution Concentration before testing yourself from memory.
Picture making a cup of tea. Add one sugar cube and it tastes mildly sweet. Add ten sugar cubes to the same amount of water and it's sickly sweet. The difference? Concentration — how much stuff is packed into a given space.
In chemistry, many reactions take place in solutions. The concentration determines how fast reactions occur and how much product you get. Too dilute and nothing happens. Too concentrated and the reaction might be dangerously fast.
Think of concentration as 'crowding'. In a concentrated solution, lots of solute particles are crowded into each cubic decimetre. In a dilute solution, only a few solute particles occupy the same space.
The key relationship is: Concentration = Mass of solute ÷ Volume of solution. Notice it's the volume of the final solution, not just the solvent you started with.
Unit Alert: Volume must be in dm³ for the concentration to be in g/dm³. Remember: 1 dm³ = 1000 cm³, so divide cm³ by 1000 to get dm³.
Let's work through the method with a typical exam question. The key is getting the units right and being clear about what each number represents.
Worked example: Calculating Solution Concentration
A student dissolves 15g of sodium chloride in water to make 500cm³ of solution. Calculate the concentration in g/dm³. [3 marks]
Answer: 30 g/dm³
Notice how we converted the volume first. This is where many students lose marks — they forget to convert cm³ to dm³ and get an answer that's 1000 times too big.
The concentration formula can be rearranged to find any of the three variables. This is crucial for different types of exam questions.
Worked example: Finding Mass of Solute
What mass of sugar is needed to make 2 dm³ of solution with concentration 40 g/dm³? [2 marks]
Answer: 80g
Concentration questions are mark-friendly if you know the common traps. Here's how to avoid the mistakes that cost students marks.
Mark Scheme Insight: Examiners give method marks for showing the formula and substitution, even if your final answer is wrong. Always show these steps clearly.
Common Trap: Questions often give you the volume in cm³ but ask for concentration in g/dm³. Students who forget to convert get answers that are 1000 times too large. If your concentration is over 1000 g/dm³, check your units.
Exam precision note: In Understanding Solution Concentration - Concentration of solutions - Use of amount of substance in relation to masses of pure substances, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.
Concentration is a relative description as well as a calculated value. A solution described as dilute simply has less solute per unit volume than another solution of the same substance, while a concentrated solution has more. There is no single GCSE cut-off such as 10 g/dm3 that always separates dilute from concentrated, because the comparison depends on context and on the substance involved. Students should therefore focus on the calculation, the units used, and whether the answer is sensible for the volume and mass given.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
To convert cm³ to dm³, divide by 1000, because 1 dm³ = 1000 cm³. Students who forget to convert the volume get a concentration in g/dm³ that is 1000 times too big.
Concentration is the amount of solute dissolved in a given volume of solution, commonly measured in grams per cubic decimetre (g/dm³). It is the volume of the final solution that counts, not just the solvent used.
Concentration in g/dm³ = mass of solute (g) ÷ volume of solution (dm³). For 15g of sodium chloride in 500cm³ of solution, the volume is 500 ÷ 1000 = 0.5dm³, so the concentration = 15 ÷ 0.5 = 30 g/dm³.
A dilute solution has less solute per unit volume than another solution of the same substance, while a concentrated solution has more. There is no single cut-off that separates dilute from concentrated, because the comparison depends on context and the substance involved.
A solute is the substance that dissolves, a solvent is the liquid that dissolves it (usually water), and a solution is the homogeneous mixture formed when a solute dissolves in a solvent.
The concentration formula can be rearranged to find any of its three variables: concentration = mass of solute ÷ volume of solution, so volume = mass ÷ concentration. The volume comes out in dm³ when the concentration is in g/dm³.
Concentration matters because many reactions take place in solutions, and the concentration determines how fast reactions occur and how much product is made. Too dilute and nothing happens; too concentrated and the reaction might be dangerously fast.
Dissolving 15g of sodium chloride to make 500cm³ of solution gives a concentration of 30 g/dm³. The volume is first converted, 500cm³ ÷ 1000 = 0.5dm³, then concentration = 15g ÷ 0.5dm³ = 30 g/dm³.
Concentration is calculated with the volume of the final solution, not just the volume of solvent at the start. The relationship is concentration = mass of solute ÷ volume of solution, with volume in dm³ to give g/dm³.
A concentration answer that is 1000 times too big usually means the volume was not converted from cm³ to dm³. If a calculated concentration is over 1000 g/dm³, the units should be checked; 1 dm³ = 1000 cm³, so cm³ are divided by 1000.