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Moles = concentration × volume in dm³, then mass = moles × Mr. Free GCSE AQA Chemistry notes with worked examples and 6 questions answered.
Calculating Moles and Mass from Solution Concentration is part of Using concentrations of solutions in mol/dm 3 (chemistry only) (HT only) in GCSE AQA Higher Chemistry.
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Moles = concentration × volume in dm³, then mass = moles × Mr. Free GCSE AQA Chemistry notes with worked examples and 6 questions answered.
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Use this lesson to stay inside Using concentrations of solutions in mol/dm 3 (chemistry only) (HT only) while connecting the detail back to the wider Topic 3: Quantitative chemistry topic.
Read the core explanations from Calculating Moles and Mass from Solution Concentration before testing yourself from memory.
You're making a sports drink. Too weak and it won't work. Too strong and it tastes awful. Getting the concentration right matters.In chemistry, we need to know exactly how much substance is dissolved in our solutions. This isn't just mixing - it's precision engineering at the molecular level.
Picture a triangle. At the top: moles of solute. Bottom left: concentration. Bottom right: volume. This triangle is your key to every concentration calculation.
Cover what you want to find. The triangle shows you what to multiply or divide. Want moles? Multiply concentration by volume. Want concentration? Divide moles by volume.
Knowing moles is useful, but labs work with grams. You need to convert between moles and mass using molar mass.
This creates a two-step process: concentration → moles → mass. First use the triangle to find moles, then multiply by molar mass to get grams.
Let's tackle the type of calculation that appears in every GCSE chemistry exam. Watch for the units - they're where most marks are lost.
Worked example: Finding Moles from Concentration
Calculate the number of moles of sodium chloride in 250 cm³ of 0.2 mol/dm³ NaCl solution. [3 marks]
Answer: 0.05 mol of NaCl
Worked example: Finding Mass from Concentration
Calculate the mass of potassium hydroxide in 100 cm³ of 0.5 mol/dm³ KOH solution. (Mr of KOH = 56) [4 marks]
Answer: 2.8 g of KOH
Concentration questions follow a predictable pattern. Master the method and you'll never drop marks on these calculations again.
Common Exam Traps: Examiners love giving volume in cm³ when the formula needs dm³. They also test whether you can link concentration to mass in one calculation.
Exam precision note: In Calculating Moles and Mass from Solution Concentration - Using concentrations of solutions in mol/dm 3 (chemistry only) (HT only) - Using concentrations of solutions in mol/dm 3 (chemistry only) (HT only), 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.
These concentration problems often have two linked steps. First, use concentration and volume to calculate the number of moles in solution. Then, if the question asks for mass, convert moles into mass using Mr. Students often mix these steps together or forget the volume conversion from cm3 to dm3. A careful method is to write the unit conversion first, calculate moles second, and only then use mass = moles x Mr. That structure makes the reasoning easier to follow and avoids 1000-fold unit errors.
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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.
The concentration triangle has moles of solute at the top, with concentration and volume at the bottom. Covering the value to be found shows the calculation: moles = concentration × volume, and concentration = moles ÷ volume, with volume in dm³.
Moles = concentration × volume, with the volume in dm³. For 250 cm³ of 0.2 mol/dm³ sodium chloride solution, the volume is 250 ÷ 1000 = 0.25 dm³, so moles = 0.2 × 0.25 = 0.05 mol.
Mass is calculated from concentration in two steps: moles = concentration × volume in dm³, then mass = moles × Mr. For 100 cm³ of 0.5 mol/dm³ KOH (Mr = 56), moles = 0.5 × 0.1 = 0.05 mol and mass = 0.05 × 56 = 2.8 g.
The most common error in concentration calculations is not converting the volume from cm³ to dm³ by dividing by 1000, which causes a 1000-fold unit error. Writing the unit conversion first, then calculating moles, then mass, avoids this.
There are 0.05 mol of sodium chloride in 250 cm³ of 0.2 mol/dm³ NaCl solution. The volume is converted first, 250 ÷ 1000 = 0.25 dm³, then moles = concentration × volume = 0.2 × 0.25 = 0.05 mol.
Molar mass is needed because concentration in mol/dm³ gives moles, but labs work with grams. Concentration and volume give the moles first, then mass = moles × molar mass converts this into grams, such as 0.05 × 56 = 2.8 g of KOH.