1. Solute, solvent and solution
The solute is the substance that dissolves. The solvent is the liquid it dissolves in. Together they make the solution. Concentration always uses the volume of the whole solution, not the volume of solvent added.
2. Concentration calculations
Convert cm3 to dm3 by dividing by 1000: 25.0 cm3 = 0.0250 dm3.
3. Concentration of ions
Use the formula to find how many ions each formula unit releases. A 0.100 mol dm−3 Na2CO3 solution has [Na+] = 0.200 mol dm−3 and [CO32−] = 0.100 mol dm−3.
4. Dilutions
This works because the moles of solute stay the same when you add water. Use it only for diluting the same solute with no reaction. V2 is the final total volume, not the volume of water added.
5. Making a standard solution (Required Practical 1)
A standard solution has an accurately known concentration. The aim is to get all the weighed solid into the flask and to make the final volume exactly right.
Method
- Weigh by difference: weigh the weighing boat with the solid, tip the solid into a beaker, then reweigh the boat. Mass transferred = first mass − second mass. Record both readings.
- Dissolve the solid in a small volume of distilled water in the beaker, stirring with a glass rod. If you warmed it to help it dissolve, let it cool to room temperature.
- Pour the solution through a funnel into a volumetric flask.
- Rinse the beaker, the glass rod and the funnel with distilled water, and add all the washings to the volumetric flask.
- Add distilled water until the bottom of the meniscus sits on the graduation line, viewed at eye level. Add the last few drops with a dropping pipette.
- Stopper the flask and invert it several times to mix.
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Apparatus for making a standard solution
Labels to include:
- Balance and weighing boat
- Beaker with glass rod
- Funnel in neck of volumetric flask
- Wash bottle of distilled water
- Graduation line on flask neck
- Meniscus viewed at eye level
- Dropping pipette and stopper
The solution and all washings pass through the funnel into the volumetric flask, which is filled to the line and inverted to mix.
Measuring volumes: use a volumetric pipette for an accurate aliquot (e.g. 25.0 cm3) rather than a measuring cylinder, which has a much larger uncertainty. Distilled water already in the volumetric flask before you start is fine, because you make up to the line afterwards.
6. Method mistakes and their effects
| Mistake | Effect | Improvement |
|---|---|---|
| Washings not added to the flask | Some solute is lost, so the concentration is lower than calculated | Rinse beaker, rod and funnel into the flask |
| Water added past the line | Solution is too dilute; pouring some out does not fix this, because the removed liquid contains solute | Start again |
| Not inverted to mix | Solution is not uniform, so samples vary | Stopper and invert several times |
| Made up while still warm | Liquid contracts on cooling, so the final volume is below the line | Cool before making up to the mark |
| Measuring cylinder used for the aliquot | Large volume uncertainty | Use a volumetric pipette |
7. Quick checks
Finesse practice: indicative answers to check your reasoning, not official mark allocations. Ar: H 1.0, O 16.0, Na 23.0.
Q1. 0.0125 mol of solute is made up to 250.0 cm³. What is the concentration?Show answer
V = 250.0 ÷ 1000 = 0.2500 dm3
c = 0.0125 ÷ 0.2500 = 0.0500 mol dm−3
Q2. What mass of NaOH is needed for 500.0 cm³ of 0.200 mol dm⁻³ solution?Show answer
n = 0.200 × 0.5000 = 0.100 mol; M = 23.0 + 16.0 + 1.0 = 40.0 g mol−1
mass = 0.100 × 40.0 = 4.00 g
Q3. 10.0 cm³ of 2.00 mol dm⁻³ solution is made up to 250.0 cm³. New concentration?Show answer
c2 = (2.00 × 10.0) ÷ 250.0 = 0.0800 mol dm−3
Q4. What is [Cl⁻] in 0.150 mol dm⁻³ MgCl₂?Show answer
Each MgCl2 gives 2 Cl−: [Cl−] = 2 × 0.150 = 0.300 mol dm−3
Q5. A student overshoots the graduation line, then removes liquid with a pipette. Why is the concentration still wrong?Show answer
The solution is already mixed, so the liquid removed contains solute. The flask now holds less solute than weighed, so the concentration is too low. The solution must be remade.
8. Sources
Sources and examiner guidance (reviewed 1 October 2026)
- Chemrevise — AQA 1.2 Calculations (amount of substance) revision guide (N. Goalby) — primary content reference (concentrations, dilution)
- AQA 7405 specification — 3.1.2 Amount of substance (incl. Required Practical 1) — 3.1.2.5 Balanced equations and calculations; Required Practical 1
- AQA 7404/1 mark scheme, June 2022 — Q02.1–02.3: mass before/after, mass → moles → concentration, fault → effect → improvement
- AQA 7404/1 examiner report, June 2022 — Q02.1: describe weighing by difference; Q02.3: explain how the fault affects the result
- AQA 7404/1 mark scheme, June 2023 — Q03.1: washings into flask, meniscus, invert
- AQA 7404/1 examiner report, June 2023 — Q03.1: where the washings go
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