Use Of Amount Of Substance In Relation To Masses Of Pure Substances

Two sacks stand side by side in a farm supply store. They hold the same mass, they cost the same money, and both are sold as nitrogen fertiliser. One of them will put more than twice as much nitrogen into the soil as the other, and nothing on the outside of either sack tells you which. The only way to find out is to read the chemical formula and do about ninety seconds of arithmetic. That arithmetic is what this lesson is about, and it is the single most useful piece of number work in the whole of chemistry.

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 farmer chooses between two sacks and how a mining company decides whether an ore is worth digging out of the ground. Then you will take that fraction off the page and put it to work on a real mass, turning a percentage into tonnes of metal or kilograms of nutrient. Three steps, one balance, and no instrument more exotic than a calculator.