Definition
Molecular weight is the sum of the weights (atomic weights) of all the atoms that make up one molecule. It tells you at a glance how heavy a single molecule is compared with others.
Add up the parts to get the weight of the whole
Imagine you built a LEGO car. To find its total weight, you just add up the weight of the 4 wheels and the body blocks, one by one. In the world of molecules, we calculate weight the very same way.
A water molecule is made of 2 hydrogen atoms and 1 oxygen atom. The relative weight of a hydrogen atom is about 1, and an oxygen atom is about 16. So the weight of a water molecule is 1 times 2, plus 16, which gives 18. This sum of the weights of all the atoms in a molecule is exactly what we call molecular weight.
Carbon dioxide works the same way. One carbon atom (12) plus two oxygen atoms (16×2=32) gives a molecular weight of 44. Even for a complicated molecule, you can find it by reading the chemical formula and adding up the atoms' weights one by one.
A bit more precisely: it's a unitless, relative number
Atoms are far too light to weigh on an ordinary scale. So scientists set the weight of one carbon-12 atom at exactly '12' and used it as the yardstick. Every other atom's relative weight, called its 'atomic weight,' is measured against it.
That means molecular weight, which is built from atomic weights, is a pure ratio with no units like grams or kilograms. Saying water's molecular weight is 18 means that if a carbon atom is 12, a water molecule weighs 18 on that same scale.
In a real lab, though, you have to handle amounts you can actually see. So scientists gather molecules into bundles of Avogadro's number (about 6.02×10²³ molecules, or 1 mole). The mass of 1 mole is almost exactly the molecular weight with 'grams (g)' attached. So 1 mole of water molecules weighs about 18 grams (more precisely, around 18.015 grams).
Molecular weight shapes a substance's properties and chemistry calculations
Molecular weight isn't just a number to calculate. It's also an important clue about how a substance behaves. In general, among molecules with similar structures, a higher molecular weight (usually because the molecule is bigger and has more electrons) means stronger attraction between molecules. That's why a higher molecular weight tends to mean a higher boiling point and melting point. But molecular weight alone doesn't decide everything. Water (18) forms hydrogen bonds, so unlike carbon dioxide (44), which is heavier, it is a liquid at room temperature.
For example, methane, the main component of natural gas, has a small molecular weight of 16 and is a gas at room temperature. Plastic polymers, with molecular weights in the tens of thousands to hundreds of thousands or more, are solids at room temperature.
Molecular weight is also essential when a chemical plant makes medicines or new materials. Just as you weigh flour and sugar when you bake, in a chemical reaction you need the molecular weight to mix the reactants in exactly the right ratio of particles and get the product you planned.
Common misconceptions
If water's molecular weight is 18, one water molecule weighs 18 grams.
Molecular weight is a unitless relative ratio. A single water molecule is incredibly light. It takes about 602 sextillion molecules (1 mole) to reach 18 grams.
Every chemical substance, like table salt (NaCl), has a molecular weight.
Salt isn't made of separate molecules. It's a crystal where ions link together in an endless lattice. Since it isn't a molecule, strictly speaking it has a 'formula weight' rather than a molecular weight.
Where you meet it
The molecular weight of water (H₂O) is about 18: hydrogen (1×2) plus oxygen (16).
The molecular weight of the oxygen gas (O₂) you breathe in is about 32.
The molecular weight of the carbon dioxide (CO₂) you breathe out is about 44.
In one sentence
Molecular weight is the sum of the relative weights of the atoms in a molecule, and it's a basic yardstick for understanding a substance's properties and doing chemistry calculations.