Weighing Accurately with an Electronic Balance

Almost every quantitative experiment in chemistry starts with a mass. Making a standard solution, finding an empirical formula, measuring a yield, working out water of crystallisation: all of them depend on weighing something correctly. Modern electronic balances make weighing look effortless, but good technique still makes the difference between an accurate result and a misleading one.

Types of balance

Balance	Typical resolution	Features	Used for
Top-pan balance	0.1 g or 0.01 g	open pan, quick	most school practicals
Precision balance	0.001 g	sometimes a small draught shield	better school and college work
Analytical balance	0.0001 g (0.1 mg)	enclosed glass draught shield	standard solutions, quantitative analysis
Microbalance	0.000001 g (1 μg) or better	very sensitive	research, pharmaceuticals

The resolution is the smallest change the display can show. The actual uncertainty of a reading is usually taken as half the resolution (for example, ±0.005 g on a 0.01 g balance), although some courses use the full resolution. Follow the convention your course uses.

Before you start

1. Check the balance is level. Most balances have a small spirit-level bubble; adjust the feet until it’s centred.
2. Place it on a firm, stable bench away from draughts, doors, windows, heaters and vibration.
3. Switch it on and let it warm up if required (analytical balances often need several minutes).
4. Check it reads zero with nothing on the pan. If not, press tare (or zero).
5. Clean the pan with a soft brush if there’s any residue.

Taring

The tare button resets the display to zero with a container on the pan, so the reading shows only the mass of what you add.

1. Put the empty weighing boat, beaker or watch glass on the pan.
2. Press tare. The display reads 0.00 g.
3. Add the substance until you reach the mass you want.

Taring is quick and convenient, but for the most accurate work, weighing by difference is better.

Weighing by difference

This is the preferred method whenever you need to know exactly how much substance ended up in your flask, for example when making a standard solution.

1. Weigh a weighing bottle (or boat) containing the substance. Record the mass.
2. Tip the substance into your beaker or flask.
3. Reweigh the bottle with any traces left in it. Record the mass.
4. Mass transferred = first mass − second mass.

Why it’s better: any solid that sticks to the weighing boat is automatically accounted for, because you’ve measured what actually left it.

Example:

• Mass of weighing bottle + solid = 15.382 g
• Mass of weighing bottle after tipping = 12.874 g
• Mass of solid transferred = 2.508 g

Good technique

• Never put chemicals directly on the balance pan. Use a weighing boat, watch glass or beaker.
• Don’t weigh hot objects. Warm objects create rising air currents that make them appear lighter, and they can damage the balance. Cool them first, ideally in a desiccator.
• Close the draught shield doors on analytical balances before reading.
• Wait for the reading to stabilise. Many balances show a symbol when the reading is stable.
• Handle containers with tongs or clean gloves for precise work; fingerprints add mass and moisture.
• Add small amounts with a spatula as you approach the target mass; tap the spatula gently.
• Record the mass immediately to the full number of decimal places shown.
• Clean up any spills at once with a brush; corrosive chemicals damage the balance.

Problems and how to fix them

Problem	Likely cause	Fix
Reading drifts up	substance absorbing water from the air (hygroscopic)	weigh quickly, use a lidded weighing bottle
Reading drifts down	substance evaporating, or object still warm	cover the container; cool the object first
Reading fluctuates	draughts or vibration	close doors, move away from vents, don’t lean on the bench
Reading jumps around with plastic boats	static electricity	use a metal or glass container, or an anti-static device
Balance doesn’t read zero	not tared, dirty pan, not level	clean, level, tare

Hygroscopic and deliquescent substances

Some substances absorb water from the air:

• Hygroscopic substances absorb moisture (for example, anhydrous copper(II) sulfate, calcium chloride).
• Deliquescent substances absorb so much that they dissolve in it (for example, sodium hydroxide pellets).

Their mass keeps increasing on the balance, so you can’t weigh them accurately. That’s why solid sodium hydroxide is not used to make standard solutions. Instead, chemists use a primary standard: a pure, stable, non-hygroscopic solid such as anhydrous sodium carbonate or potassium hydrogenphthalate. See acid–base titration.

Heating to constant mass

When you heat a substance to drive off water or decompose it (for example, finding the formula of a hydrate), you need to be sure the reaction is complete:

1. Weigh the empty crucible (with lid).
2. Add the sample and weigh again.
3. Heat, allow to cool (in a desiccator), and weigh.
4. Repeat heating, cooling and weighing until two consecutive masses agree within the balance’s uncertainty, for example within 0.01 g.

That’s constant mass, and it confirms that no more water or gas is being lost. See hydrates and water of crystallisation.

Weighing liquids

For accurate masses of liquids:

1. Tare a stoppered container.
2. Add the liquid with a dropping pipette.
3. Stopper it to prevent evaporation, then read.

Weighing is often more accurate than measuring volume for viscous liquids, which cling to glassware.

Percentage uncertainty

percentage uncertainty = (uncertainty ÷ mass measured) × 100

Example 1: a 0.01 g balance (uncertainty ±0.005 g), weighing 0.25 g: 0.005 ÷ 0.25 × 100 = 2.0%

Example 2: the same balance, weighing 2.50 g: 0.005 ÷ 2.50 × 100 = 0.20%

Example 3: when weighing by difference, two readings are involved, so the uncertainty doubles: ±0.01 g. For 2.508 g: 0.01 ÷ 2.508 × 100 = 0.40%

The lesson: weighing larger masses gives smaller percentage uncertainties. If an experiment gives a large percentage error from weighing, redesign it to use more material, or use a more precise balance. For combining uncertainties, see calculating percentage uncertainty.

Calibration

Balances are calibrated using certified reference masses (calibration weights). Many analytical balances have a built-in calibration routine. In schools, a technician usually checks balances regularly. If a balance gives consistently wrong readings with a known mass, report it rather than using it.

Mass vs weight

Strictly, balances measure mass (in grams or kilograms), although in everyday speech we talk about “weighing”. Weight is a force (in newtons) that depends on gravity. Electronic balances are calibrated to show mass at their location, which is another reason they need calibrating after being moved long distances.

Key takeaways

• Level the balance, keep it away from draughts, and tare before adding substance.
• Weighing by difference gives the most accurate transferred mass.
• Never weigh hot objects or put chemicals straight on the pan.
• Hygroscopic substances such as NaOH can’t be weighed accurately; use primary standards instead.
• Heat to constant mass to confirm a reaction is complete.
• Larger masses give smaller percentage uncertainties.