Equations, formulae and the mole

Last time

Before we start: last time, in Protons, neutrons, electrons and isotopes, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

How chemists count

A single drop of water holds more molecules than there are stars in our galaxy. Chemists cannot count atoms one by one, so they count in a huge unit called the mole. Once you know the amount in moles, equations tell you exactly how much of everything reacts and forms.

The mole

One mole is the amount of substance containing 6.02 times ten to the twenty-three particles, the Avogadro constant, as there are atoms in twelve grams of carbon-12. Amount in moles equals mass over molar mass. For solutions, amount equals concentration in moles per dm cubed, times volume in dm cubed. For gases at room temperature and pressure, amount equals volume in dm cubed over twenty-four.

Balancing equations

A balanced equation has the same number of each kind of atom on both sides, and the same total charge. Sodium chlorate(I) reacts with hydrogen peroxide to give sodium chloride, water and oxygen: NaClO plus H two O two goes to NaCl plus H two O plus O two. Check: Na one, Cl one, H two, O: one plus two is three on the left; one plus two is three on the right. It is balanced. State symbols are added in brackets: s, l, g, aq.