Experiment: Preparing a standard solution

A standard solution is a solution whose concentration is known accurately, and preparing one correctly is a core practical skill in the Form 4 Acids, Bases and Salts chapter. It is also the starting point for titration, so getting every step right here pays off later. This guide sets out the method the way the marking scheme expects, then works through the science process skills the practical papers reward. Our teachers use exactly this sequence in one-to-one lessons because each step removes one common source of error.

Aim

To prepare a standard solution of a known concentration, for example sodium carbonate solution, using a volumetric flask so that the number of moles of solute in a fixed volume of solution is accurately known.

Apparatus and materials

• Volumetric flask (for example, a 250 cm³ flask with a single calibration mark)
• Electronic balance
• Weighing bottle or weighing boat, and a spatula
• Beaker and glass stirring rod
• Filter funnel
• Wash bottle of distilled water
• Dropper (teat pipette)
• The solid solute, for example anhydrous sodium carbonate

Procedure

1. Weigh the empty weighing bottle on the electronic balance and record its mass.
2. Add the solid solute to the weighing bottle and record the new mass, so the mass of solid used is known by difference.
3. Tip the solid into a clean beaker. Rinse the weighing bottle with a little distilled water and pour the rinsings into the beaker so no solid is left behind.
4. Add distilled water to the beaker and stir with the glass rod until the solid has completely dissolved.
5. Using the filter funnel, transfer the solution carefully into the volumetric flask.
6. Rinse the beaker, the glass rod and the funnel several times with distilled water, adding every rinsing to the flask, so that all of the dissolved solute is transferred.
7. Add distilled water until the level is just below the calibration mark.
8. Using the dropper, add distilled water drop by drop until the bottom of the meniscus rests on the calibration mark, viewed at eye level.
9. Stopper the flask and invert it several times to mix the solution thoroughly and make the concentration uniform.

Expected observations

The solid dissolves completely to give a colourless solution (the colour depends on the solute; sodium carbonate gives a colourless solution). At the end, the bottom of the meniscus sits exactly on the calibration mark when the flask is viewed at eye level, and the mixed solution is clear with no undissolved particles.

Inference and conclusion

Because the mass of solute and the final volume of solution are both known accurately, the concentration can be calculated: the number of moles equals the mass divided by the molar mass, and the molarity equals the number of moles divided by the volume in cubic decimetres. The solution is therefore a standard solution, its concentration is known accurately, and it can be used with confidence in titration or dilution. The key idea is that every particle of the weighed solute must reach the flask, and the volume must be read precisely, or the stated concentration will be wrong.

Science process skills (Paper 3 style)

Making an operational definition. Define a standard solution operationally: it is a solution prepared by dissolving a known mass of solute and making it up to a known, fixed volume in a volumetric flask, so that its concentration is accurately known.

Identifying variables in a linked investigation. If the experiment were extended to compare the concentration produced by different masses of solute in the same flask, the manipulated variable would be the mass of solute weighed, the responding variable would be the concentration (moles per cubic decimetre) of the solution, and the controlled variables would be the volume of the volumetric flask and the temperature.

Tabulating data. Record the mass of the weighing bottle before and after, the mass of solid by difference, and the volume of the volumetric flask, each with its correct unit in the column heading.

Communicating. State a conclusion that links the measured mass and fixed volume to the calculated concentration, and explain why the answer is only reliable if the transfer and the reading of the meniscus were done correctly.

Safety precautions

• Wear safety goggles, because some solutes are irritant and splashes must be kept out of the eyes.
• Handle the solid with a spatula and do not touch it with bare hands, so the skin is not irritated and the mass is not contaminated.
• Wipe up any spilled solid or solution at once, so the bench stays clean and the balance readings are not affected.
• Add water carefully near the mark, because an overfilled flask cannot be corrected and the whole preparation must be repeated.

Common errors

• Reading the meniscus with parallax. Looking from above or below the mark gives the wrong volume; always read the bottom of the meniscus at eye level.
• Overshooting the calibration mark. Adding water past the mark makes the solution too dilute; if this happens, the preparation must be started again.
• Not rinsing the beaker and rod. Solute left behind lowers the true concentration below the intended value.
• Adding water to a hot solution before making up. If dissolving released or absorbed heat, let the solution reach room temperature before making up to the mark, because volume changes with temperature.
• Weighing on a dirty or unbalanced balance. Zero the balance first, or the mass of solute, and therefore the concentration, will be wrong.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we rehearse this method until the sequence is automatic: weigh by difference, dissolve fully, transfer with rinsing, make up to the mark at eye level, then mix. Because the same discipline underlies every titration in SPM Chemistry, mastering it now makes the rest of the Acids, Bases and Salts practical work far more secure, and it is exactly the kind of procedure tested in the Paper 3 practical assessment (Paper 3 is a practical test assessing science process skills).