Measuring Mass with a Balance

Introduction

Almost every chemistry experiment begins with measuring out a substance, and most of the time that means using a balance. A careful measurement gives reliable results; a careless one can ruin an experiment before it starts. This page explains how laboratory balances work, what their readings mean and the techniques chemists use to measure mass accurately.

Core explanation

Types of balance. Schools and laboratories mainly use electronic balances (also called top-pan balances). A sensor under the pan detects the downward force of the sample and the balance converts it into a mass reading. Older beam balances compare the unknown mass with known masses on a lever. Very precise analytical balances sit inside a glass case that stops air currents disturbing the reading.

Resolution. Each balance shows readings to a fixed number of decimal places. A balance reading to 0.01 g has a resolution of 0.01 g; an analytical balance might read to 0.0001 g. You should record every digit the balance shows — if it displays 2.40 g, write 2.40 g, not 2.4 g, because the final zero tells the reader how precise the measurement was.

Taring. Samples are rarely placed straight on the pan. Instead, a container such as a weighing boat or beaker is put on the pan and the tare (zero) button is pressed. The display returns to 0.00 g, so when the sample is added only its mass is shown.

Weighing by difference. For accurate work, chemists weigh a container with the sample, tip the sample into the apparatus, and then weigh the container again. The mass transferred is the difference between the two readings. This method accounts for any powder left clinging to the container.

Good technique. - Place the balance on a level, steady bench away from draughts. - Check the display reads zero before you start. - Never put chemicals directly on the pan. - Close any draught shield and wait for the reading to settle. - Let hot objects cool first: warm air rising from them pushes up on the pan and gives a falsely low reading. - Clean up spills immediately.

Step-by-step reasoning

To measure out about 2 g of salt using a balance reading to 0.01 g:

1. Check the balance is level and reads 0.00 g. 2. Place a clean weighing boat on the pan and press tare. 3. Add salt a little at a time with a spatula until the reading is close to 2 g. 4. Wait for the reading to settle and record it exactly, for example 2.03 g. 5. Transfer the salt, then reweigh the boat if you need the mass by difference.

Visual explanation

Reading Value (g) --- --- Mass of boat + salt before transfer 5.87 Mass of boat after transfer 3.84 Mass of salt transferred 5.87 − 3.84 = 2.03

The table shows weighing by difference. In the measurement simulation, repeated weighings of the same object cluster tightly around one value on a good balance, showing high precision.

Real-world analogy

Taring is like resetting a car's trip meter to zero before a journey. The car's total distance is still stored, but the trip meter shows only the distance you add from now on. The tare button lets the balance show only the mass you add.

Real-world example

Jewellers weigh gold on precise balances because the price depends on mass. Many countries require these balances to be tested and certified regularly by government inspectors, so customers can trust that a 10.00 g chain really contains 10.00 g of gold.

Why?

Why must hot objects be cooled before weighing? Hot objects warm the air around them. Warm air rises, creating a small upward current that pushes on the pan and container, so the balance reads less than the true mass. Waiting until the object is at room temperature removes this error.

Common misconception

Students sometimes believe that a balance showing more decimal places is always "more accurate". More decimal places means better resolution (and usually better precision), but a balance can still be inaccurate if it is not level or has not been calibrated. Accuracy means being close to the true value.

Worked example

Question: A beaker has a mass of 48.26 g. After a student adds copper sulfate crystals the reading is 53.71 g. What mass of crystals was added?

Reasoning: Mass of crystals = total mass − mass of beaker = 53.71 − 48.26.

Answer: 5.45 g of copper sulfate crystals.

Quick check

1. What does pressing the tare button do? Answer: It resets the display to zero with the container on the pan, so the next reading shows only the mass of the sample added.

Exam focus

Practical exam questions often give two balance readings and ask for the mass by difference. Record masses to the same number of decimal places as the balance and include the unit. You may also be asked to suggest a source of error — draughts, an unlevel balance, a hot sample or spilled solid are good answers.

Advanced insight

An electronic balance really measures a force, so it is calibrated using certified standard masses where it is installed. High-precision work also corrects for buoyancy : the air pushes up very slightly on the sample and on the standard masses, and the correction matters when they have very different densities.

Summary

Electronic balances measure mass quickly; their resolution shows how finely they can read. Use a container and the tare function, record every displayed digit with the unit, and use weighing by difference for accurate transfers. Keep balances level, clean and away from draughts, and let hot items cool before weighing.

Practice questions

1. A balance reads to 0.01 g. How should a mass of exactly four grams be recorded? Answer: 4.00 g. 2. An empty dish weighs 12.40 g; with a sample it weighs 15.05 g. Find the sample's mass. Answer: 15.05 − 12.40 = 2.65 g. 3. Give two ways a student could make a balance reading unreliable. Answer: Any two: weighing on an unlevel bench, a draught near the balance, weighing a hot object, not zeroing the balance, putting chemicals straight on the pan. 4. Why is weighing by difference more accurate than weighing the sample once? Answer: It accounts for any sample left behind in the container, so it gives the mass actually transferred.