Add relative atomic masses to get the relative formula mass of a substance, then use the mole as a fixed count of particles so a mass on the balance can be turned into a number of particles.
You can now put a name to a structure from a card of measurements. Every recovery job on the bench ends with a different question: how much. How much copper came out of the cable, how much acid went into the bath. A balance answers in grams, because grams are what a balance can see. But a reaction is a count of particles meeting each other, one at a time, and grams are not counts. Two jars that weigh the same almost never hold the same number of particles, because the particles inside them weigh different amounts. This stop builds the bridge between the two.
Start with the formula. Add up the relative atomic masses of every atom in it and you get the relative formula mass, written Mr. Water is H₂O: two hydrogens at 1 each and one oxygen at 16, so 2 + 16 = 18. Sodium chloride is NaCl, so 23 + 35.5 = 58.5. Brackets multiply everything inside them. Calcium hydroxide is written Ca(OH)₂, meaning one calcium and two lots of OH, so 40 + 2 x 17 = 74.
Now the bridge. One mole of anything contains about 6.02 x 10^23 particles. That number is so large it has a name of its own, Avogadro's number, and it never changes: a mole of water and a mole of iron hold the same count. The useful part is what a mole weighs. One mole of a substance weighs its Mr in grams. One mole of water is 18 g. One mole of sodium chloride is 58.5 g. Weigh out the Mr in grams and you have counted out 6.02 x 10^23 particles without counting a thing.
Calcium carbonate is CaCO₃, so its Mr is 40 + 12 + 3 x 16 = 100, and one mole weighs 100 g. How many moles are there in 20 g? Divide the mass by the Mr: 20 / 100 = 0.2 moles. In particles that is 0.2 x 6.02 x 10^23, or about 1.2 x 10^23 of them. The relation comes in three forms and you need all three. Moles equals mass divided by Mr. Mass equals moles multiplied by Mr. Mr equals mass divided by moles. Which one you reach for depends on which two of the three the question hands you.
One mole of water and one mole of carbon dioxide hold exactly the same number of molecules. They do not weigh the same, because a carbon dioxide molecule is heavier than a water molecule: Mr 44 against Mr 18. So a mole of carbon dioxide is 44 g while a mole of water is 18 g. The mole counts particles, not grams. Same count, different mass. Turn it around and the same is true: two jars of equal mass hold different counts whenever their formula masses differ, and the lighter particle is always the one there is more of.
Add up the relative atomic masses in a formula and you have its relative formula mass, remembering that a bracket multiplies everything inside it. One mole of anything holds about 6.02 x 10^23 particles and weighs its Mr in grams. Moles equals mass divided by Mr, and the same relation rearranges to give either of the other two. Equal masses of two substances with different formula masses hold different counts, because their particles weigh different amounts.
Adapted from OpenStax Chemistry 2e (CC-BY 4.0), openstax.org · license
Last week ended with a name for every fragment in the crate. This week the club weighs one. A tub of white powder on the balance reads 20 g, and the label says calcium carbonate, CaCO₃.
Using Ca = 40, C = 12 and O = 16, what is its relative formula mass?
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