You cannot count molecules one by one. There are far too many, and they are far too small. So chemists count them the way a bank counts coins: by weighing. That one idea, turning a weight into a count, sits behind almost every solution made in a lab.
A mole is just a very big dozen
A dozen is 12 of anything. A mole is a counting word too, just a far bigger one: about 602 thousand billion billion of anything. The useful part is that every substance has a known weight per mole, its molar mass. Weigh out a substance, divide by its molar mass, and you know how many moles, how many molecules, you have.
Weigh what is really in the bottle
Here is where real lab mistakes happen, and it is never the arithmetic. Many chemicals come with extra water locked inside their crystals. Copper sulfate, the bright blue crystals, is usually sold this way. If you use the weight per mole of the dry form when the bottle holds the watery form, picture this: you make a solution that is about a third weaker than its label, and every result that uses it is wrong by the same amount.
The same goes for purity. If a material is 98% pure, you weigh a little more to get the amount you need. The question to ask before every weighing is: how much of what I am weighing is the thing I actually want?
Recipes work in counts, not weights
A chemical reaction is like a recipe that says "two of these for every one of those". It counts molecules, not grams. So to use it, you always go through the count: weight to moles, moles to moles using the recipe, then back to weight or volume.
Which ingredient runs out first
Mix equal weights of two ingredients and one will usually run out long before the other, because equal weights are rarely equal counts. Mix a gram of hydrogen with a gram of oxygen, for example, and the oxygen runs out first, with most of the hydrogen left over. The one that runs out first decides how much can be made.
Why this matters to you
Medicines, fertilisers, water treatment, food additives: they are all made from recipes that count molecules by weighing them. Most mistakes are not in the maths. They are in the first number: the wrong weight per mole because nobody checked what was really in the bottle.
Want the full lesson, with worked calculations, practice problems and the questions?
Read the full lesson → Free: sign in with your email, no password.
References
- BIPM, The International System of Units (SI), 9th edition (2019).
- Restek, "Are your reference standards weight corrected for purity?" (FAQ).
- D. Harvey, Analytical Chemistry 2.1, chapter 2 (LibreTexts).