The Mole Concept PDF

The Mole Concept A famous Italian scientist, Amedeo Avogadro, devised a method for chemists to count atoms and molecules. If we weigh out elements in the same proportions as their relative atomic masses, we will always have the same number of atoms of these elements. Element...

The Mole Concept A famous Italian scientist, Amedeo Avogadro, devised a method for chemists to count atoms and molecules. If we weigh out elements in the same proportions as their relative atomic masses, we will always have the same number of atoms of these elements. Element Mass Carbon 12 g Magnesium 24 g Aluminium 27 g Calcium 40 g Iron 56 g Copper 63.5 g Silver 198 g All of the elements listed above contain the same number of atoms. This number of atoms is 6 x 1023. This number is called Avogadro’s number or Avogadro’s Constant in honour of Amedeo Avogadro and is given the symbol L. **Definition: One mole of a substance is the amount of that substance that contains 6 x 1023 particles of that substance.** Converting Moles to Grams **Definition: Mass of one mole of an element = Relative Atomic Mass in grams.** Element Symbol Relative Atomic Mass Hydrogen H 1 Carbon C 12 Nitrogen N 14 Oxygen O 16 Sodium Na 23 Sulphur S 32 Chlorine Cl 35.5 Calcium Ca 40 Manganese Mn 55 Iron Fe 56 Copper Cu 63.5 For example, One mole of Sodium is 23 grams. One mole of Oxygen is 35.5 grams etc. What is the mass of three moles of aluminium atoms? 3 moles Al = ? grams Al 1 mole of Al = 27 g 3 moles of Al = 27 x 3 3 moles of Al = 81 grams 1 By adding up all the relative atomic masses of all the atoms in a molecule of a compound we can obtain the relative molecular mass of the compound. **Definition: The relative molecular mass of a compound is the average mass of one molecule of that compound compared with one twelfth of the mass of one atom of the carbon-12 isotope.** What is the relative molecular mass of water, H2O? Rel. Atomic Mass of 2 H = 2 x 1 = 2 Rel. Atomic Mass of 1 O = 1 x 16 = 16 Relative Molecular Mass = 18 One mole of H2O = 18 grams **Definition: Mass of one mole of a compound = Relative Molecular Mass in grams.** What is the mass of one mole of hydrated copper(II) sulfate, CuSO4.5H2O? Cu = 63.5 S = 32 4 O = 4 x 16 = 64 5H2O = 5 x 18= 90 Rel. Molecular Mass = 249.5 One mole of CuSO4.5H2O = 249.5 grams Converting Grams to Moles 𝑀𝑎𝑠𝑠 𝑜𝑓 𝑒𝑙𝑒𝑚𝑒𝑛𝑡 No. of moles of an element = 𝑟𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝑎𝑡𝑜𝑚𝑖𝑐 𝑚𝑎𝑠𝑠 How many moles are there in 36 g of carbon atoms? 12 g Carbon = 1 mole 1 1 g Carbon = 12 36 36 g Carbon = 12 36 g Carbon = 3 moles 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑 Number of moles of a compound = 𝑟𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 How many moles of water molecules are there in 117 g of water? Calculate the relative molecular mass of water O = 16 2H=2x1 =2 Relative molecular mass = 18 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 117 No. moles of water = 𝑅𝑒𝑙. = 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 18 Answer = 6.5 moles 2  𝑀𝑎𝑠𝑠 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑢𝑏𝑠𝑡𝑎𝑛𝑐𝑒 Number of moles of a substance = 𝑀𝑎𝑠𝑠 𝑜𝑓 𝑜𝑛𝑒 𝑚𝑜𝑙𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑢𝑏𝑠𝑡𝑎𝑛𝑐𝑒 How many moles of sulfuric acid, H2SO4, are there in 12.25 g of sulfuric acid? Calculate the relative molecular mass of sulfuric acid, H2SO4 2H=2x1 =2 1 S = 1 x 32 = 32 4 O = 4 x 16 = 64 Rel. Molecular Mass = 98 𝑀𝑎𝑠𝑠 𝑜𝑓 𝐻2𝑆𝑂4 12.25 No. moles of H2SO4 = 𝑀𝑎𝑠𝑠 𝑜𝑓 𝑜𝑛𝑒 𝑚𝑜𝑙𝑒 𝑜𝑓 𝐻2𝑆𝑂4 = = 0.125 98 Answer = 0.125 mole Calculations involving Avogadro’s Number One mole of a substance contains 6 x 1023 particles of that substance. Converting Moles to Number of Atoms or Molecules How many atoms of sodium are present in 0.25 moles of this metal? 0.25 moles Na = ? atoms 1 mole of sodium contains 6 x 1023 atoms 0.25 x 6 x 1023 = 1.5 x 1023 atoms 0.25 moles Na = 1.5 x 1023 atoms Converting Grams to Number of Atoms or Molecules How many atoms are there in 28.75 g of sodium? 1 mole of sodium contains 6 x 1023 atoms 6 𝑥 1023 1 g of sodium contains 23 28.75 𝑥 6 𝑥 1023 28.75 g Na contains atoms = 7.5 x 1023 23 Answer = 7.5 x 1023 3 Converting Numbers of Atoms or Molecules to Moles How many moles of aluminium atoms are there in 9 x 1023 atoms of aluminium? 6 x 1023 atoms of Al = 1 mole 1 1 atom of Al = 6 𝑥 1023 mole 9 𝑥 1022 9 x 1022 atoms of Al = 6 𝑥 1023 mole = 0.15 mole Answer = 0.15 mole Converting Numbers of Atoms or Molecules to Grams How many grams of zinc are present in 3 x 1023 atoms of zinc? 1 mole of Zn has a mass of 65 grams 65 1 atom of zinc has a mass of 6 𝑥 1023 g 65 𝑥 3 𝑥 1020 3 x 1020 atoms have a mass of 6 𝑥 1023 Answer = 0.0325 g 4