Mass: The Kilogram

Introduction

Every SI base unit has a simple name — metre, second, mole — except one. The base unit of mass is the kilo-gram, which already contains the prefix "kilo", meaning a thousand. This odd situation is a leftover from history. In chemistry, mass is probably the quantity you will measure most often, whether weighing a solid on a balance or working out how much product a reaction forms, so it pays to understand its unit well.

Core explanation

Why the prefix? When the metric system was created, the gram was chosen as the basic unit of mass, defined as the mass of one cubic centimetre of water. But the gram is small — about the mass of a paperclip — and the metal standard needed to be a practical size. A 1 kg cylinder was made instead, and later scientists building a coherent system of units found the kilogram fitted better with the metre and second. So the kilogram became the base unit, even though its name includes a prefix.

A strange consequence for prefixes. Prefixes are never doubled up, so we do not write "millikilogram". Instead, prefixes for mass are always attached to the gram: 1 mg = 10⁻³ g, 1 μg = 10⁻⁶ g, and 1 Mg = 10⁶ g (one tonne).

From a metal cylinder to a constant. From 1889 to 2019 the kilogram was defined as the mass of a single platinum–iridium cylinder, the International Prototype Kilogram, kept in a vault near Paris. Careful comparisons showed that it and its official copies were drifting apart by tens of micrograms over a century. Nobody could say which was "right". In 2019 the kilogram was redefined by fixing the value of the Planck constant , h = 6.626 070 15 × 10⁻³⁴ J s. Specialised instruments called Kibble balances compare mechanical and electrical power to realise the kilogram from this constant.

Mass and weight. Mass is the amount of matter in an object, measured in kilograms. It is the same everywhere. Weight is the force of gravity on that mass, measured in newtons (N). On Earth, weight ≈ mass × 9.8 N/kg. An astronaut has the same mass on the Moon but only about one-sixth of the weight. Laboratory "weighing" really measures mass, because balances are calibrated using known masses.

Useful conversions.

Unit In kilograms --- --- 1 tonne (t) 1000 kg 1 kilogram (kg) 1 kg 1 gram (g) 0.001 kg 1 milligram (mg) 0.000 001 kg

In school chemistry, masses are usually measured in grams, because reacting amounts are typically a few grams.

Step-by-step reasoning

To find the mass of a solid sample on a top-pan balance:

1. Make sure the balance is level and reads zero. 2. Place an empty weighing boat on the pan and press "tare" to reset the reading to zero. 3. Add the solid carefully until the target mass is reached. 4. Wait for the reading to settle, then record every digit shown, with the unit g.

Visual explanation

Imagine an old two-pan balance. On one side sits the object; on the other, standard masses are added until the beam is level. A modern electronic balance hides this comparison inside, but the principle is the same: the unknown mass is compared with a known standard.

Real-world analogy

A kilogram is like a shop's official "one-pound" price tag: every other price is judged against it. Replacing the metal prototype with the Planck constant is like replacing a paper price list that could fade with a rule that can never change.

Real-world example

Pharmaceutical factories weigh active ingredients in milligrams. A single tablet may contain only a few milligrams of the medicine, mixed with hundreds of milligrams of inactive filler. Accurate mass measurement is therefore vital to patient safety.

Why?

Why did the prototype kilogram need replacing? Because a physical object can gain mass by absorbing contaminants from the air or lose mass when cleaned. Over a century, small changes built up. A definition based on a fixed constant of nature cannot change, so the kilogram is now stable forever.

Common misconception

"Mass and weight mean the same thing." In everyday speech they are used interchangeably, but in science mass (kg) is the amount of matter and weight (N) is a force. Your mass would be the same on the Moon, but your weight would be much less.

Worked example

Question: A student weighs out 250 mg of sodium chloride. Express this mass in grams and in kilograms.

Reasoning: 1 g = 1000 mg, so 250 mg = 250 ÷ 1000 = 0.250 g. 1 kg = 1000 g, so 0.250 g = 0.250 ÷ 1000 = 0.000 250 kg.

Answer: 0.250 g, which is 2.50 × 10⁻⁴ kg.

Quick check

1. Why is the kilogram unusual among the seven base units? Answer: It is the only base unit whose name already contains a prefix (kilo).

Exam focus

Be ready to convert between t, kg, g and mg, and to distinguish mass from weight clearly: mass in kg, weight in N. Remember that in chemistry calculations, such as moles = mass ÷ molar mass, mass is normally used in grams.

Advanced insight

The Kibble balance balances the weight of a test mass against an electromagnetic force produced by a coil in a magnetic field. By measuring currents and voltages in terms of the Planck constant, it links mass to fixed constants. An alternative route counts atoms in a near-perfect sphere of silicon-28, linking the kilogram to the Avogadro constant.

Summary

The kilogram is the SI base unit of mass and the only one with a prefix in its name. Mass prefixes are attached to the gram, not the kilogram. Since 2019 the kilogram has been defined by fixing the Planck constant, replacing a metal cylinder that drifted. Mass is the amount of matter; weight is the gravitational force on it, measured in newtons.

Practice questions

1. Convert 3.5 kg into grams. Answer: 3500 g. 2. Why is "millikilogram" never used? Answer: Prefixes cannot be combined; mass prefixes are attached to the gram instead, and one thousandth of a kilogram is simply one gram (g). 3. An object has a mass of 10 kg. What is its weight on Earth, taking g as 9.8 N/kg? Answer: 10 × 9.8 = 98 N. 4. Which constant of nature is now used to define the kilogram? Answer: The Planck constant, h.