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3.3 the mole

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3.3 the mole

  1. 1. The Mole Section 3.3
  2. 2. The Mole • The mole is simply a chemist’s measurement for the amount of a substance. • It is a very large number (6 x 1023) as it ‘counts’ very small ‘objects’ such as atoms and molecules which have a very small masses. • This rather strange number was not selected deliberately – instead, it is the number of atoms in a sample of any element that has a mass in grams that is numerically equal to the elements atomic mass. • It allows chemists to work with masses in the lab which are feasible.
  3. 3. The Mole Definition A mole of any substance is defined as the amount of the substance that contains as many particles as 12 g of carbon – 12.
  4. 4. (i) (ii) How many molecules are there in 0.5 mols of chlorine gas? How many atoms are there in 2 mols of water? (iii) How many electrons are there in 1.5 mols of Calcium? (i) 1 mol of Cl2 6 1023 molecules 0.5 mols of Cl2 (ii) 0.5 6 1023 1 mol of H2 0 6 1023 molecules [Cl2 is composed of molecules] 3 1023 molecules [H2 O is composed of molecules] 2mols of H2 0 2 6 1023 1.2 1024 molecules Each molecule of water contains 3 atoms 1.2 1024 molecules contains 3 1.2 1024 (ii) 3.6 1024 atoms. 1 mol of Ca 6 1023 atoms [Ca is composed of atoms] 1.5 mols of H2 0 1.5 6 1023 9 1023 atoms Each atom of Calcium contains 20 electrons 9 1023 atoms contain 23 9 1024 2.07 1025 electrons.
  5. 5. Molar Volume • One mole of any gas should occupy the same volume as one mole of any other gas at the same conditions of temperature and pressure. • Gases are often compared at STP, standard temperature and pressure. Standard pressure = 101325 Pa Standard temperature = 273 K • Molar Volume = 22.4 L = 22,400 cm3 = 2.24 x 10–2 m3
  6. 6. (i) What is the volume in litres at STP of 0.01 mols of methane (CH4 ) gas? (ii) How many mols are there in 280 cm3 of fluorine gas at STP? (i) 1 mol of CH4 22400 cm3 0.01mols of CH4 (ii) 0.01 22400 224 cm3 1 mol of F2 22400 cm3 1 mols of F2 = 1cm3 22400 1 280 mols of F2 = 280cm3 22400 0.0125 mols of F2 280cm3
  7. 7. Relative Molecular Mass The relative molecular mass is the average mass of a molecule relative to one twelfth the mass of the carbon 12 atom. The relative molecular mass has no units as it is the ratio of two masses.
  8. 8. (i) Calculate the relative molecular mass of (a) sulfuric acid, (H2 SO4 ) (b) Oxygen, (O2 ). (i) (a) H2 SO4 (b) O2 2(1) 32 4(16) 98 2(16) 32
  9. 9. Molar Mass • The molar mass of a substance is the mass in grams of one mole of the substance. • The molar mass has the same numerical value as its relative molecular mass, but its units are grams (g). Substance Relative molecular mass Molar mass Sulfuric aicd, H2 SO4 98 98 g Glucose, C6H12 O6 180 180 g Oxygen, O2 32 32 g Sulfur dioxide, SO2 64 64 g 1 mol of carbon, 12 g
  10. 10. (i) How many moles are there in 990 g of cabon dioxide (CO2 ) (ii) The daily intake of calcium for an adult is 800 mg, how many moles of calcium is this? (i) 1 mol of CO2 44 g 1 mols of CO2 1g 44 1 990 mols of CO2 990 g 44 22.5 mols of CO2 990 g (ii) 1 mol of Ca 40 g 1 mols of Ca 1g 40 1 800 10 3 mols of Ca 800 10 3 g 40 0.02 mols of Ca 800 10 3 g [800mg 800 10 3 g]
  11. 11. More Mole Calculations Volume of X Number of particles of X (if a gas at STP) in litres at STP Moles of X x molar mass ÷ molar mass Mass of X in g Notice that there is no direct link from particles to grams, you must first convert to moles. Likewise going from volume to mass.
  12. 12. What is the mass of one atom of calcium? 1 mol of Ca 40 g 1 mol of Ca 6 1023 atoms 6 1023 atoms 40 g 1 atom 40 6 1023 6.67 10 23 g
  13. 13. The Nissan Micra 1.5 Diesel SVE is quoted to have a CO2 emission figure of 120 g / km. For a 40 km round trip to work calculate: (i) (ii) (iii) the mass of CO2 produced. the number of moles of CO2 produced. the volume of CO2 produced at room temperature and pressure. [Molar volume at room temperature and pressure 24.0 litres] (i) 120 40 4800 km (ii) no. of mols (iii) 1mol of CO2 actual mass molecular mass 4800 no. of mols mols 44 no. of mols 109.09 mols 24.0 L 109.09 mols of CO2 109.09 mols of CO2 109.09 24L 2594.16 L
  14. 14. How many iron atoms should be consumed daily to meet the recommended daily intake of iron in the diet of 0.014 g? no. of mols acutalmass molecular mass 0.014 56 no. of mols 2.5 10 4 mols no. of mols 1mol 6 10 3 atoms 2.5 10 4 mols 2.5 10 4 2.5 10 4 mols 1.5 10 0 6 10 3 atoms atoms [H2007, Q4 (e)]

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