You’re likely to have been taught the classical definition of matter in school, which is anything that occupies space (volume) and has mass, which is sometimes interchangeable with weight.
You may also have learned the difference between mass and weight, and why the former is more important in science than the latter. This is particularly true in chemistry. Substances are typically measured in terms of molar mass when we talk about chemical reactions.
On a macroscopic scale, mass and weight seem interchangeable, especially if you’re referring to objects on the surface of the earth. However, these units of measurements start to diverge once you consider the gravitational attraction of objects.
Although the concepts of mass and weight may appear interchangeable, they actually have precise definitions. Volume, on the other hand, has a simpler and less nuanced definition as we’ll discover later on.
Understanding the concepts and measurement of volume, weight, and mass, whether on a macroscopic scale or at molecular and atomic levels, will give you a better grasp of the principles of chemistry. It also has crucial implications for practical applications such as formulating medicines.
In this post:
How is Mass Different from Weight and Volume?
Mass is the quantity of matter in an object.
It can also refer to the total number of protons and neutrons in an atom, otherwise known as the atomic mass.
Weight is the amount of force necessary to accelerate an object with mass, while volume is the space occupied by matter.
Both weight and volume are dependent on the amount of matter (mass) in an object, but they’re also limited by other factors.
Volume, weight, and mass are measured as follows:
- Volume – cubic metre
- Mass – kilogram
- Weight – the Newton, derived from 1 kg-m/s2. A Newton is the force necessary to accelerate an object with a mass of one kilogram by one metre per second, per second.
Volume
Volume is how much space a substance occupies. On a microscopic level, it can be thought of as the average distance, movement, and configuration of the particles of a substance relative to each other. Its stability depends on the state of matter. Solids, for example, have a very stable volume and shape. Liquids have unstable shapes but stable volumes, while gases have both unstable shapes and unstable volumes.
The SI unit of volume is the cubic metre, but other units such as litre, millilitre, cubic millimetre, and cubic centimetre are also used in chemistry. For very small particles like colloids, radius rather than volume is measured. Common units include micrometres and nanometres.
The volume of a contained substance will remain relatively the same at given constants. For example, the volume of oil inside a hydraulic piston can’t be compressed. Air inside a sealed bottle, on the other hand, will fully occupy the space inside and can be compressed. Volume simply refers to the space occupied by matter.
However, volume also depends on other factors, such as the state of matter, pressure, and temperature. Air expands under high temperatures, for instance.
The Equation for Volume in Chemistry
As previously mentioned, both temperature and pressure affect the volume of a substance. In solids and liquids, the volume change is trivial or negligible. However, the volume of gases can change drastically depending on the temperature and pressure.
The relationship between the volume, temperature, and pressure in a gas is explained by the following gas laws:
- Boyle’s law – the volume of a gas is inversely proportional to the pressure. Gas contracts as pressure increases.
- Charles’ law – volume is directly proportional to temperature. Gas expands as temperature increases.
- Avogadro’s law – gas volume is directly proportional to the amount of gas.
Mass
The simplest definition of mass is how much matter there is in a given substance.
The more matter that’s packed into a substance, the greater its mass. Mass generally correlates with how heavy a substance is. Physically, mass is related to the concept of inertia, or how much an object resists a change in motion.
This is set out in the Newtonian first law of motion, which states an object that is not moving will stay at rest and an object in motion will keep moving unless it is acted upon by an unbalanced force. The force required to counteract inertia must be greater than the mass of an object. More massive objects require greater force to change their state of motion.
How to Calculate Mass in Chemistry
You can measure the mass of a substance with weighing scales. Precise chemical reactions are also possible with precise measurements of mass.
There are several ways to calculate mass in chemistry. The first one is to derive it from the known density and volume. To do this, you multiply the volume by density.
m = v x d
For example, pure gold has a density of 19.3 g/cm3. A pure gold bar that has a dimension of 4.2 cm x 2.4 cm x 0.2 cm or 2.016 cm3 has a mass of:
2.016 cm3 x 19.3 g/cm3 = 38.91 g
In chemistry, mass can also refer to molar mass. You can calculate a substance’s molar mass by first determining the chemical formula. Then, multiply the atomic mass of each element contained in the substance by the number of atoms the element has, which you’ll find in the chemical formula. Finally, add the results and express the total in gram units.
For example, you would calculate the molar mass of hydrogen peroxide like this:
H2O2 = (1×2) + (16 x 2) = 34 grams
A molar mass or mole has the number of particles equal to the Avogadro’s number, or 6.022 x 1023. This is important in determining the proportions of reactants in grams.
What is Weight in Chemistry?
Weight is directly related to the force of gravity but it’s still proportional to mass. For example, an astronaut who weighs 100 kg on earth at sea level will only weigh 16.5 kg on the moon. This is because the moon’s gravity is 16.5% that of earth. Nonetheless, the mass of the astronaut remains the same. 
The mass of any given object is relatively constant, but its weight may fluctuate depending on the gravitational force. A one-kilogram rock at sea level will weigh less on top of a mountain, for instance
