Use Of Amount Of Substance In Relation To Masses Of Pure

A white crystalline solid arrives in a laboratory with nothing written on the jar. Nobody knows what it is. Within an hour an analyst can hand back its formula, and the only instrument that really mattered was a balance. That is a strange claim on the face of it: a balance reports masses, and a formula is a statement about how many atoms of each kind are locked together. Getting from one to the other is the single most useful piece of arithmetic in the whole of chemistry, and it is what this lesson is about.

You will start by learning to add up a formula, turning a string of symbols and subscripts into one number that stands for the mass of the whole unit. From there you will work out what fraction of a compound is the element you actually care about, which is how a food label turns into a sodium figure and how a mining company decides whether an ore is worth digging. Then comes the reverse trip, and it is the best part: given nothing but masses measured on a bench, you will reconstruct the formula of a substance you have never met, then sharpen that formula until it names one compound and one compound only. Both of the first two skills are worth marks on both tiers of the chemistry paper, and the third carries the single longest calculation on it.