The Mole: Formulae and Chemical Equations: Atoms, Molecules and Moles

2.0 The Mole: Formulae and Chemical Equations

2.2 Atoms, Molecules and Moles

In relative atomic and molecular masses, we do not specify units - because they are for comparison only (not actual). However, in experiments, we deal with actual masses in grams (g). How do we bring in g?

Think of this. In the relative masses: 1 for H, 12 for C, 14 for N, 16 for O, 27 for Al, 35.5 for Cl, and 56 for Fe, there are the same number of atoms, which is 1.

That being true, 1g of H, 12 g of C, 14 g of N, 16 g of O, 27 g of Al, 35.5 g of Cl and 56 g of Fe and so on also have the same number of atoms (but not 1 this time). It is a large number. These masses, which have the same number of atoms as there are in 12 g of carbon-12 isotope, are called a mole.

Similarly, 2 g of hydrogen gas (H₂), 28 g of nitrogen gas (N₂), 32 g of oxygen gas (O₂), and 71 g of chlorine gas (Cl₂), and so on are one mole of molecules each. A mole is a quantity. Its unit is mol.

Notice that, adding the unit gram (g) to RAM or RMM of a substance makes it a mole. But this is not the definition. It is important for calculations.

Questions 2.2

1. Define a mole.
2. State the mass of one mole of (a) lithium atoms (b) sodium atoms (c) magnesium atoms (d) sulphur atoms (RAM: Li =7; Na = 23; Mg = 24; S = 32).
3. Find the number of moles of atoms in (a) 2.4 g of carbon (b) 8 g of sulphur.
   (RAM: C = 12; S = 32)


4. Find the mass of (a) 0.5 moles of oxygen molecules and (b) 2 moles of magnesium.
   (RAM: O = 16; Mg = 24)


5. Determine the number of moles of molecules in (a) 0.32 g of oxygen gas (b) 90 g of water.
   (RAM: H = 1; O = 16)