Mole concepts

• Relative atomic mass (Ar)


  The average mass of one atom of an element compared to 1/12 of the mass of one carbon-12 atom
• Relative atomic mass has no unit
• Relative atomic mass of an element can be found in the Periodic Table
• Relative molecular mass (Mr)


  The average mass of one molecule of an element or compound compared to 1/12 of the mass of one carbon-12 atom

  
• Relative molecular mass is the sum of the Ar of all the atoms in a molecule
• Relative molecular mass has no unit
• Relative formula mass


  A more accurate way to refer to the Mr of ionic compounds, since ionic compounds do not exist as molecules
• Relative formula mass is calculated in exactly the same way as relative molecular mass

  
• Relative formula mass has no unit

  
• Calculating relative molecular mass

  • Mr of carbon dioxide CO2 = 12 + (2 × 16) = 44
  • Mr of sodium chloride NaCl = 23 + (1 × 35.5) = 58.5
  • Mr of hydrated magnesium sulfate, MgSO4 • 7H2O = 24 + 32 + (4 × 16) + (7 x 18) = 246
• Mole (mol)


  Used to count the number of particles (atoms, molecules, ions and even sub-atomic particles such as electrons)

  
• One mole of any substance will always contain 6.02 × 1023 particles of that substance

  
• This is also known as the Avogadro's constant
• Avogadro's Number


  • 1 mol of hydrogen atoms contains 6.02 × 1023 hydrogen atoms
  • 1 mol of water contains 6.02 × 1023 water molecules
  • 1 mol of CuSO4 contains 6.02 × 1023 formula units of CuSO4
  • 1 mol of Na+ ions contains 6.02 × 1023 Na+ ions
• • 1 mol of O2 contains 2 mol of oxygen atoms
  • 1 mol of O3 contains 3 mol of oxygen atoms
• Molar mass


  • The mass of one mole of atoms of an element (g/mol)
  • The mass of one mole of molecules of a substance (g/mol)
  • The mass of one mole of formula units of an ionic compound (g/mol)

  
• Calculating moles and molar mass

  • Number of moles of iron = 196 g / 56 g/mol = 3.5 mol
  • Number of moles of NH3 = 4.25 g / 17 g/mol = 0.25 mol
  • Mass of 0.5 mol of MgO = 0.5 mol × 40 g/mol = 20 g

  
• Avogadro's Law


  Equal volumes of all gases, under the same temperature and pressure, contain the same number of particles, hence have the same number of moles
• One mole of any gas occupies a volume of 24 dm3 at room temperature and pressure

  
• Calculating moles and volume of gases

  • Number of moles of CO2 in 3 dm3 = 3 dm3 / 24 dm3/mol = 0.125 mol
  • Volume of 0.75 mol of CH4 = 0.75 mol × 24 000 cm3/mol = 18 000 cm3
  • Volume of 4 g of O2 = (4 g / 32 g/mol) × 24 dm3/mol = 3 dm3

  
• Concentration of a solution

  • The amount of solute (in g or mol) dissolved in 1 dm3 of a solution
  • Can be expressed in g/dm3 or mol/dm3
• Calculating concentration of a solution

  • Concentration of 9.6 g NaOH in 100 cm3 = 9.6 g / 0.1 dm3 = 96 g/dm3
  • Concentration of 9.6 g NaOH in 100 cm3 = 96 g/dm3 / 40 g/mol = 2.4 mol/dm3
• Balanced chemical equation


  Provides information on reactants, products, mole ratio, and physical state of each reactant and product
• Stoichiometry


  The relationship between the number of moles of reactants and the number of moles of products involved in a chemical reaction
• Procedure for chemical calculations

  1. Construct a balanced chemical equation

  2. Find the number of moles of the given substance

  3. Compare the mole ratio of the given substance to that of the unknown substance

  4. Calculate the number of moles of unknown substance
• The volume of one mole of any gas is 24 dm3 at room temperature and pressure (r.t.p.)
• Information provided by a balanced chemical equation

  • Reactants
  • Products
  • Mole ratio
  • Physical state of each reactant and product
• A balanced chemical equation enables us to calculate the mass, volume and number of moles of reactants or products
• Mole ratio


  The ratio of the number of moles of the reactants and products
• Molar mass


  The mass of one mole of a substance
• Steps to calculate the mass of magnesium chloride formed

  1. Find the number of moles of magnesium

  2. Refer to the balanced chemical equation to find the mole ratio of magnesium to magnesium chloride

  3. Calculate the mass of magnesium chloride produced
• Steps to calculate the mass of oxygen combined with 46g of sodium in sodium oxide

  1. Find the number of moles of sodium

  2. Use the mole ratio of sodium to oxygen in the compound formula

  3. Calculate the number of moles and mass of oxygen
• Steps to calculate the maximum mass of potassium that can be extracted from 188g of potassium oxide

  1. Find the number of moles of potassium oxide

  2. Use the mole ratio of potassium oxide to potassium in the compound formula

  3. Calculate the number of moles and mass of potassium
• Avogadro's law


  The volume of a gas is proportional to the number of moles of the gas, and vice versa
• Steps to calculate the volume of ammonia gas produced

  1. Find the mole ratio of nitrogen to ammonia from the balanced equation

  2. Use the mole ratio to calculate the volume of ammonia produced from a given volume of nitrogen
• Steps to calculate the volume of oxygen gas required for complete combustion of methane

  1. Find the mole ratio of methane to oxygen from the balanced equation

  2. Use the mole ratio to calculate the volume of oxygen required from a given volume of methane
• Titration


  A method of volumetric analysis used to determine the concentrations of solutions
• Steps to solve a titration problem

  1. Write the balanced chemical equation

  2. Calculate the number of moles of the acid used

  3. Find the concentration of the alkali in mol/dm3

  4. Convert the concentration in mol/dm3 to g/dm3
• Example 1


  1. Write the balanced chemical equation for this reaction

  2. Calculate the number of moles of hydrochloric acid used

  3. Find the concentration of sodium hydroxide in mol/dm3

  4. Find the concentration of sodium hydroxide in g/dm3
• Concentration


  Solution concentration in mol/dm3 or g/dm3