WJEC Chemistry for AS Level Student Book: 2nd Edition (Draft)

WJEC Chemistry for AS Level

Since bromine only has two isotopes: peak A ( m / z 158) must be due to ( 79 Br − 79 Br) + and peak C ( m / z 162) must be due to ( 81 Br − 81 Br) + . Therefore peak B (m/z 160) is due to ( 79 Br − 81 Br) + or ( 81 Br − 79 Br) + . Since the ratio of peaks A:B:C is almost 1 : 2 : 1 it follows that natural bromine consists of a nearly 50:50 mixture of 79 Br and 81 Br. molecule 79 Br − 79 Br 79 Br − 81 Br or 81 Br − 79 Br 81 Br − 81 Br probability 1 2 × 1 2 1 2 × 1 2 or 1 2 × 1 2 1 2 × 1 2 Amount of substance In chemical reactions, for all the reactants to change into products, the correct quantity of each reactant must be used. Since the atoms that make up the reactants rearrange to form the products, it would be really useful to be able to work out these quantities on the basis of the number of atoms that react. If you pay coins into a bank, the clerk won’t count them individually – he knows the mass of a certain number of coins, e.g. £10 worth of 50p, so he will weigh a bag of coins to confirm that the correct number of coins is present. Similarly, atoms are too small to be counted individually, so they are counted by weighing a collection of them where the mass of a particular fixed number of atoms is known. Since carbon-12 is the standard chosen for relative atomic mass, the number of atoms in exactly 12g of carbon-12 is chosen as the standard and is known as the mole . The number of atoms in a mole is very large. Using a mass spectrometer, the mass of a single carbon-12 atom has been found to be 1.993 × 10 −23  g therefore the number of atoms per mole = mass per mole of 12 C mass of one atom of 12 C  = 6.02 × 10  23  mol −1 This is called the Avogadro constant , N A , after the nineteenth-century Italian chemist Amadeo Avogadro. The mass of one mole of a substance is called the molar mass , M . It has the same numerical value as A r or M r but has the unit gmol −1 . The amount of substance in moles ( n ), the mass ( m ) and molar mass ( M ) are linked by the equation: amount in moles ( n ) =  mass ( m ) molar mass ( M ) The expression for amount in moles may be rearranged in two ways to find the mass of a sample or molar mass of a substance.  =  12gmol −1 1.993 × 10 −23  g ⎧ ⎨ ⎩ 1 4 2 4 1 4

Key terms One mole  is the amount of any substance that contains the same number of particles as there are atoms in exactly 12g of carbon-12. The Avogadro constant  is the number of atoms per mole. Molar mass  is the mass of one mole of a substance.

Knowledge check

5

Calculate the mass of sodium that contains (a) 6 × 10 23 atoms (b) 4 × 10 25 atoms (Assume that the Avogadro constant is 6 × 10 23 mol –1 )

Exam tip Learn the equations that connect amount of substance and mass for a solid: n  =  m M DRAFT or  m  =  nM or  M  =  m n 36