Planning experiments and assessing risks at A2

A planning checklist

1. Aim and the measurement that answers it.

2. Variables: independent (what you change), dependent (what you measure), control (what you keep the same, and how).

3. Apparatus with sizes and resolutions (250.0 cm³ volumetric flask, 50 cm³ burette, 3 d.p. balance).

4. Quantities: calculate them so that measurements fall in a sensible range (titres of about 20 to 30 cm³; gas volumes within the syringe).

5. Method as numbered steps with enough detail to follow.

6. Processing: what you will calculate or plot.

7. Safety: hazards and specific precautions.

Calculating quantities

• Mass of solid for a standard solution: mass = c x V x M.
• Mass of sample for a target titre: work backwards from the titre to moles in the aliquot, scale up to the whole solution, then multiply by M.

A2 hazards

• Concentrated ammonia: corrosive vapour; fume cupboard.
• Phenylamine and phenol: toxic, absorbed through the skin; gloves, fume cupboard.
• Ethanoic anhydride and acyl chlorides: corrosive, react with water giving acid fumes; fume cupboard.
• Potassium manganate(VII) and dichromate(VI): oxidising; dichromate is also a carcinogen (serious health hazard symbol).
• Organic solvents (ethanol, propanone): flammable; water bath, no flames.
• Silver and copper compounds: harmful to aquatic life; do not pour down the sink.

GHS hazard symbols you should recognise.

Worked example

Calculate the mass of ammonium iron(II) sulfate, (NH4)2Fe(SO4)2·6H2O, needed to make 250.0 cm³ of a 0.0500 mol dm^-3 solution.

1. n = c x V = 0.0500 x 0.2500 = 0.0125 mol
2. M = 392.0 g mol^-1
3. mass = 0.0125 x 392.0

Answer: 4.90 g

Check yourself

1. What mass of ammonium iron(II) sulfate (M = 392.0) is needed to make 250.0 cm³ of 0.0500 mol dm^-3 solution?
   Show answer

   4.90 g. 0.0500 x 0.2500 x 392.0 = 4.90 g.

2. You want 25.0 cm³ portions of an ammonium iron(II) sulfate solution (made up to 250.0 cm³) to need about 25.00 cm³ of 0.0200 mol dm^-3 KMnO4. What mass of the salt (M = 392.0) should be dissolved?
   Show answer

   9.80 g. n(MnO4-) = 5.00 x 10^-4; n(Fe2+) per portion = 2.50 x 10^-3; total 0.0250 mol; mass = 9.80 g.

3. In an experiment to find how temperature affects the rate of a clock reaction, which is a control variable?
   • The temperature
   • The time for the colour to appear
   • The concentrations and volumes of the reactants
   • The rate
   Show answer

   The concentrations and volumes of the reactants. Temperature is independent; time (and so rate) is dependent.

4. In the same experiment, what is the dependent variable?
   • The temperature of the water bath
   • The time for the colour to appear
   • The volume of starch
   • The total volume
   Show answer

   The time for the colour to appear. It is what you measure.

5. Which precaution is most appropriate for handling phenylamine?
   • Keep it away from water because it is explosive
   • Heat it with a Bunsen burner to evaporate it
   • Store it with oxidising agents
   • Wear gloves and use a fume cupboard because it is toxic and absorbed through the skin
   Show answer

   Wear gloves and use a fume cupboard because it is toxic and absorbed through the skin. Phenylamine is toxic by skin contact and inhalation.

6. Which hazard symbol, often missed by candidates, applies to potassium dichromate(VI) because it can cause cancer?
   • Flammable (flame)
   • Serious health hazard (person with a starburst on the chest)
   • Gas under pressure
   • Explosive
   Show answer

   Serious health hazard (person with a starburst on the chest). The October 2024 report noted the third symbol, for serious health hazards such as carcinogenicity, was often missed.

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Sources

• Pearson Edexcel IAL Chemistry (2018) specification
• Pearson IAL Chemistry teacher practical guide
• Pearson examiners' report, WCH16 October 2024

Last updated 8 November 2026. Spotted a mistake? Tell us.