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1.1.4 Amount of Substance / The Mole You will be able to… • Explain the terms amount of substance, mole and the Avogadro constant. • Define and use the term molar mass. • Carry out calculations involving masses using the amount of substance in moles. The Mole Derived from the Greek word Mole meaning large heap The Mole • The chemical mole is a heap of atoms or molecules Dalton • In 1808 Dalton proposed that the atoms of each element had a characteristic atomic weight and it was atoms that were the combining units in chemical reactions to make compounds • Dalton had no method of measuring atomic weights unambiguously, so made the incorrect assumption that in the most common compound between two elements, there was one atom of each element e,g water was made from one atom of hydrogen and one of oxygen, oxygen gas contained only one atom of oxygen Gay-Lussac • Gay-Lussac studied the chemical reactions of gases and found that the ratios of volumes of the reacting gases were small integer numbers e.g two volumes of hydrogen react with one of oxygen to produce two volumes of water Lorenzo Romano Amadeo Carlo Avogadro Avogadro • In 1811, Avogadro clearly drew the distinction between a molecule and an atom • He pointed out that Dalton had confused the concepts of atoms and molecules. The Dalton ‘atom’ of Oxygen gas was in reality a molecule containing two atoms • Avogadro also suggested that ‘equal volumes of all gases at the same temperature and pressure contain the same number of molecules’ This is known as Avogadro’s Principle • Avogadro was Italian so everybody ignored his work until Stanisalo Cannizarro showed how it could be used to determine molar and indirectly atomic mass at the Karlsuhe Conference 1860 The Mole Argument • From atomic masses one atom of magnesium is twice as heavy as one atom of carbon (Mg = 24 and C = 12). Therefore… • If one atom of Mg is twice as heavy as one atom of C then 1000 atoms of Mg are twice as heavy as 1000 atoms of C etc. • A piece of Mg twice as heavy as a piece of C must contain the same number of atoms e.g 2 tonnes of Mg contains the same number of atoms as 1 tonne of Carbon 12g C 24g Mg 56g Fe 108g Ag 197g Au Extend this to molecules • Molecules are made up of atoms in different ratios • The relative molecular mass of a molecule in grams contains one mole of that specific molecule 16g CH4 17g NH3 18g H2O 44g CO2 34g H2S Definitions One Mole is the amount of substance containing the same number of units there are in exactly 12g of 12C The substance must be specified and may be atoms, molecules, ions, crystals grains of sand, flies sheep, trucks… anything! Avogadro’s Constant is the number of units in a mole It is given the symbol L and is 6.0221415 x 1023 So What? • What is difficult to grasp is the size of Avogadro’s number, 6.0221415 x 1023 that is… 602 214 150 000 000 000 000 000 Basically if you can see a particle of it with your eye you wont be able to gather a mole of it on this earth Mole Calculations (I) The number of moles of a substance is defined as the amount of substance, n. To calculate n just divide the mass of the substance, m, by the molar mass, M n = mass Molar mass = m M = g g mol -1 = mol Mole Calculations (II) To calculate the number of atoms or molecules = n x L Number of atoms or molecules = mass Molar mass x Avogadro’s Constant = mL = nL M = g g mol -1 x mol -1 Mole Problems Relative atomic masses: H = 1; O = 16; Na = 23; Cl = 35.5; Ca = 40; Cu = 64 1. 2. 3. 4. 5. 6. What is the mass of 4 mol of sodium chloride, NaCl? How many moles is 37g of calcium hydroxide, Ca(OH)2? How many moles is 1kg of calcium? What is the mass of 0.125 mol of copper oxide, CuO? 0.1 mol of a substance weighs 4g. What is the weight of 1 mole? 0.004 mol of a substance weighs 1g. What is the relative formula mass of the compound? n =m M