Laboratory Balances and Mass Measurements

A laboratory balance measures the mass of a sample, and the main types are the top-loading balance (typically precise to 0.01 g or 0.001 g) and the analytical balance (typically precise to 0.0001 g). In brief: to weigh a chemical, place a container on the balance, press tare to set zero, add the chemical and read the stable display; never put chemicals directly on the pan.

Mass is one of the measurements chemists make most often, and almost every quantitative experiment depends on it. This article describes the types of balances, the steps for accurate weighing, methods such as weighing by difference, the sources of error, a worked example with data and a short history of weighing.

What types of balance are there?

Readability is the smallest change the display can show. The real accuracy also depends on calibration and conditions.

How do you weigh a sample correctly?

1. Check that the balance is on a level, stable surface, away from drafts and vibration. Many balances have a bubble level.
2. Switch it on and let it warm up if required, and make sure the display reads zero.
3. Place a clean, dry container (weighing boat, paper or beaker) on the pan and close the draft shield doors on analytical balances.
4. Press tare (zero) so that the display shows 0.000 g.
5. Add the sample slowly with a spatula until you reach the desired mass.
6. Wait until the reading is stable, then record all the digits shown, with units.
7. Remove the sample and container, and clean up any spill at once with a brush.

What is weighing by difference?

Weighing by difference is used when a substance cannot be placed directly on the pan, or when an exact amount delivered into another container is needed. The method works as follows:

1. Weigh the container with the sample and record this mass, m₁.
2. Pour or scoop some sample into the receiving vessel without weighing it there.
3. Weigh the container again and record the mass, m₂.
4. The mass transferred is m₁ − m₂.

Using the same balance for both readings cancels many small errors, since any constant error is the same in both readings.

What can cause errors in weighing?

• Air currents: drafts from windows or doors move the pan. Close the draft shield.
• Hot or cold objects: a hot crucible heats the air around the pan, which creates upward air currents, making the object seem lighter. Let samples cool to room temperature in a desiccator.
• Moisture: some substances absorb water from the air and gain mass while on the pan. Weigh them quickly in a closed container.
• Static electricity: plastic containers or dry powders can build up charge and cause drifting readings.
• Unlevel balance or vibrations: give inaccurate readings.
• Fingerprints: oils from the skin add mass to precise objects. Use tongs or gloves.

Worked example: recording and using mass data

A student weighs a sample of sodium chloride on a balance with readability of 0.001 g.

1. Direct weighing. A weighing boat is tared, and 2.345 g of salt is added. The record is 2.345 g. All three decimal places are recorded, including zeros, so 2.340 g would be written as 2.340 g.
2. Weighing by difference. A bottle with salt has a mass of 18.762 g. After pouring out some salt, the mass is 16.419 g. The mass transferred is 18.762 − 16.419 = 2.343 g.
3. Moles. The molar mass of NaCl is 58.44 g/mol, so 2.343 g contains 2.343 ÷ 58.44 = 0.04009 mol.
4. Making a solution. If this salt is dissolved to make 250.0 mL of solution, the concentration is 0.04009 ÷ 0.2500 = 0.1604 mol/L.
5. Relative error from the balance. A readability of 0.001 g on 2.343 g gives a relative uncertainty of up to 0.001 ÷ 2.343 × 100% = 0.04%, which is small. On 0.020 g it would be 5%, so larger samples give better relative precision.

How do you care for a balance?

• Never overload the pan beyond its capacity.
• Keep it clean, and wipe spills immediately with a soft brush or a damp cloth after unplugging.
• Do not move it once it is installed and calibrated, unless necessary.
• Calibrate it regularly with certified reference masses.
• Place hygroscopic, corrosive or volatile chemicals in a closed container.

How did weighing develop?

The equal-arm balance has been used for thousands of years, with stone and metal weights. Careful weighing changed chemistry in the late 1700s, when Antoine Lavoisier used precise balances to show that mass is conserved in reactions. Electronic balances, based on electromagnetic force compensation, became common in laboratories from the 1970s onward, and today they are the standard instrument. Since 2019 the kilogram is defined by the Planck constant, and a device called the Kibble balance links mass to electrical measurements.

Which mistakes are common?

Mistake 1: Weighing chemicals directly on the pan

This contaminates and can corrode the pan. Use a container or paper.

Mistake 2: Dropping the readout digits

If the balance shows 1.500 g, write 1.500 g, not 1.5 g. The last digits carry information about precision.

Mistake 3: Reading before the display is stable

Wait until the stability indicator appears.

Mistake 4: Weighing warm objects

Let hot items cool to room temperature first.