IC5.1.4 Calculations © Oxford University Press Calculations

IC5.1.4 Calculations © Oxford University Press This equation links moles, mass, and molar mass. If you have a known mass of a substance (element or compound), you can use this equation to work out the number of moles of the substance. number of moles = mass ÷ molar mass

IC5.1.4 Calculations © Oxford University Press The equation can also be written like this.

IC5.1.4 Calculations © Oxford University Press For example, suppose you had 36 g of water. Its molar mass is 18 g/mol. So you would have 36 ÷ 18 = 2 mol of water. number of moles = mass ÷ molar mass

IC5.1.4 Calculations © Oxford University Press If you know the number of moles you have of a compound, you can also work out the mass of each element. number of moles = mass ÷ molar mass

IC5.1.4 Calculations © Oxford University Press For example, suppose you had 1 mol of carbon dioxide molecules, CO 2.  You would have 1 mol of carbon atoms and 2 mol of oxygen molecules.  This is 1 × 12 = 12 g of carbon atoms and 2 × 16 = 32 g of oxygen atoms. If you add 12 g and 32 g together, you get 44 g. This is the mass of 1 mol of carbon dioxide molecules.

IC5.1.4 Calculations © Oxford University Press Here’s another example. What is the molar mass of magnesium hydroxide, Mg(OH) 2 ?  A r of Mg = 24, A r of O = 16, and A r of H = 1.  M r of Mg(OH) 2 = × (16 + 1) = = 58  So the molar mass is 58 g/mol.

IC5.1.4 Calculations © Oxford University Press Try these calculations yourself. number of moles = mass ÷ molar mass A r of C = 12 1.How many moles of carbon are there in 6 g of carbon?  Number of moles = 6 g ÷ 12 g/mol = 0.5 mol  6 g = 0.5 mol

IC5.1.4 Calculations © Oxford University Press number of moles = mass ÷ molar mass A r of Na = 23, O = 16, H = 1 2.How many moles of compound are there in 20 g of sodium hydroxide, NaOH?  M r = = 40  Number of moles = 20 g ÷ 40 g/mol = 0.5 mol  20 g = 0.5 mol