Measuring Mass

Why this skill matters

Mass measurement is essential for stoichiometry calculations, percentage yield determinations, and rates experiments where mass loss is monitored. On Paper 5 and 6, examiners check whether you record masses to the correct precision, use taring properly, and understand sources of error in weighing.

Method and technique

Using a top-pan balance

1. Place the balance on a flat, level surface away from draughts and vibrations.
2. Switch on and wait for the display to settle at 0.00 g (for a 2 d.p. balance).
3. If weighing a substance: place a suitable container (beaker, watch glass, weighing boat) on the pan.
4. Press the tare/zero button. The display resets to 0.00 g.
5. Add the substance to the container until the desired mass is reached.
6. Record the mass to the full precision of the balance (e.g., 4.52 g, not 4.5 g).

Weighing by difference

For transferring a precise mass (e.g., in a preparation):

1. Weigh the container with the substance: record as m1.
2. Transfer the substance to the reaction vessel.
3. Reweigh the container with any residue: record as m2.
4. Mass transferred = m1 - m2.

This method is more accurate than trying to add exactly the right mass, because any substance that sticks to the container is accounted for.

Monitoring mass loss

For rate-of-reaction experiments (e.g., marble chips and acid):

1. Place the flask and reactants on the balance.
2. Start the reaction and record the mass at regular time intervals (every 30 seconds).
3. The mass decreases as gas escapes. Plot mass loss against time.
4. Use cotton wool in the flask neck to prevent spray loss while allowing gas to escape.

Common errors and how to avoid them

Not taring: Forgetting to tare means the container mass is included. If you cannot tare, weigh the container separately and subtract, but taring is faster and reduces calculation errors.

Recording to wrong precision: A balance reading 12.5 g when the balance shows two decimal places should be recorded as 12.50 g. The trailing zero shows you used a 2 d.p. balance, which is a marking point.

Draughts affecting readings: Air currents cause the reading to fluctuate. Close the balance door (if enclosed) or shield from draughts.

Not using weighing by difference: When transferring a solid to a solution, some always sticks to the container. Weighing by difference accounts for this and gives the true mass transferred.

Hot objects on the balance: Convection currents from a hot object cause the reading to drift. Allow objects to cool before weighing.

Marking points examiners look for

• Tare the balance before adding the substance
• Record mass to the full precision of the balance
• Use weighing by difference for accurate mass transfer
• Allow hot objects to cool before weighing
• Shield from draughts
• State the precision of the balance (e.g., +/- 0.01 g)