How to Measure Mass Using a Balance

How to Use a Scale or Balance

A balance is an instrument used to measure mass in a laboratory.
A balance is an instrument used to measure mass in a laboratory. Matthias Tunger / Getty Images

Mass measurements in chemistry and other sciences are performed using a balance. There are different types of scales and balances, but two methods can be used on most instruments to measure mass: subtraction and taring.

Key Takeaways: Measure Mass Using a Balance

  • A balance or scale is an instrument used to measure mass in the science laboratory.
  • One common method of measuring mass is to tare the scale and measure the mass directly. For example, this is how people weigh themselves.
  • The other common method is to place a sample into a container and measure the mass of the container plus sample. The mass of the sample is obtained by subtracting the mass of the container.

Proper Use of a Balance

Before using a balance, it's important to take some preliminary steps. This will help ensure you get the most accurate and precise measurement.

  • Be sure you understand how to use a balance before taking mass measurements.
  • The balance should be clean and free of debris.
  • The balance should be on a level surface.
  • Never place a sample directly on the balance. You should use a weighted boat, weighing sheet, or another container to hold the sample. Some chemicals you may use in a lab could corrode or otherwise damage the surface of the weighing pan. Also, make certain your container will not chemically react with your sample.
  • If the balance has doors, be certain to close them before taking a measurement. Air movement impacts the accuracy of mass measurements. If the balance does not have doors, make certain the area if free of drafts and vibrations before measuring mass.

Mass by Difference or Subtraction

If you place a container full of sample and weigh it, you're getting the mass of both the sample and the container, not just the sample. To find the mass:

mass of sample = mass of sample/container - mass of container

  1. Zero the scale or press the tare button. The balance should read "0".
  2. Measure the mass of the sample and container.
  3. Dispense the sample into your solution.
  4. Measure the mass of the container. Record the measurement using the correct number of significant figures. How many this is will depend on the particular instrument.
  5. If you repeat the process and use the same container, do not assume its mass is the same! This is particularly important when you are measuring small masses or are working in a humid environment or with a hygroscopic sample.

Mass by Taring

When you use the "tare" function on a scale, you are making certain the reading starts from zero. Usually, there is a labeled button or knob to tare the balance. With some instruments, you need to manually adjust the reading to zero. Electronic devices do this automatically, but require periodic calibration.

  1. Zero the scale or press the tare button. The scale reading should be "0".
  2. Place the weighing boat or dish on the scale. There is no need to record this value.
  3. Press the "tare" button on the scale. The balance reading should be "0".
  4. Add the sample to the container. The value given is the mass of your sample. Record it using the proper number of significant figures.

Sources of Error

Whenever you take a mass measurement, there are several potential sources of error:

  • Air gusts may push the mass up or down.
  • Buoyancy can affect measurements. Buoyancy is directly proportional to the air volume that is displaced and is affected by air density changes due to temperature and pressure fluctuations.
  • Condensation of water on cold items may increase apparent mass.
  • Dust accumulation can add to mass.
  • Evaporation of water from damp items may change mass measurements over time.
  • Magnetic fields may affect components of the scale.
  • Temperature changes can cause components of the balance to expand or contract, so a measurement taken on a hot day may differ from one taken on a cold day.
  • Vibration may make it difficult to obtain a value, as it will fluctuate.

Is It Mass or Weight?

Remember, a balance gives you a mass value. The mass would be the same whether you measured it on the Earth or on the Moon. On the other hand, the weight would be different on the Moon. While it's common to use the terms mass and weight interchangeably, they are only the same values on Earth!

Sources

  • Hodgeman, Charles, Ed. (1961). Handbook of Chemistry and Physics, 44th Ed. Cleveland, USA: Chemical Rubber Publishing Co. pp. 3480–3485.
  • Rossi, Cesare; Russo, Flavio; Russo, Ferruccio (2009). Ancient Engineers' Inventions: Precursors of the Present. History of Mechanism and Machine Science. ISBN 978-9048122523.