Preparing a Standard Solution for H2 Chemistry Paper 4

Example: prepare 250.0 cm3 of $0.100 \text{ mol dm}^{-3}$ sodium carbonate solution.

$$V = 250.0 \text{ cm}^3 = 0.2500 \text{ dm}^3$$

$$m = 0.100 \times 0.2500 \times 106.0 = 2.65 \text{ g}$$

Record the mass to match the balance resolution, usually to two decimal places if using a 0.01 g balance.

2 | Weighing by difference

Weighing by difference reduces loss errors when transferring a solid.

This is better than weighing onto paper and trying to brush every crystal into the beaker. If the solid is hygroscopic, cap the weighing bottle quickly and record the transfer promptly.

3 | Dissolving and quantitative transfer

Procedure:

1. Add the weighed solid to a small beaker.
2. Add about half the final volume of deionised water.
3. Stir until every crystal dissolves.
4. Pour through a clean funnel into the volumetric flask.
5. Rinse the beaker, stirring rod, and funnel with small portions of deionised water.
6. Add all washings to the volumetric flask.
7. Fill near the mark, then use a dropping pipette for the final meniscus adjustment.
8. Stopper and invert at least ten times until the solution is uniform.
9. Label the flask with solute, concentration, date, and hazard if relevant.

Never make to the mark before the solid has dissolved. Undissolved solid can dissolve later, changing the volume and concentration.

4 | Making to the mark

Read the meniscus at eye level. The bottom of the meniscus should sit on the calibration line for colourless aqueous solutions.

If you overshoot the mark, do not remove liquid with a pipette and pretend the concentration is correct. The solution has already been diluted too far. In an exam write-up, state that the solution must be remade.

After stoppering, invert the flask repeatedly. Swirling a volumetric flask is not enough because dense solution can remain near the neck or base.

5 | Dilution factor

For dilution from a stock solution:

$$C_1V_1 = C_2V_2$$

If 25.0 cm3 of stock is pipetted into a 250.0 cm3 volumetric flask and made to the mark:

$$\text{dilution factor} = \frac{250.0}{25.0} = 10.0$$

The diluted concentration is one tenth of the stock concentration.

For serial dilution, multiply the dilution factors at each step. State the aliquot volume and final flask volume for every stage.

6 | Uncertainty and common mistakes

Mention volumetric flask tolerance when discussing uncertainty. A 250.0 cm3 Class A flask has a small but non-zero uncertainty, so the concentration cannot be exact even if the mass is weighed carefully.

7 | Related guides

• H2 Chemistry volumetric practical deep dive
• H2 Chemistry qualitative analysis workflow
• H2 Chemistry experiments hub

Sources

1. SEAB - H2 Chemistry (Syllabus 9476) GCE A-Level 2026
2. SEAB - H2 Chemistry Specimen Paper 4 (9476/04), first exam 2026

Contact us to learn more about our tailored packages and available pricing options.