Chemistry calculations

• Relative mass


  Different elements have different masses because they have different numbers of protons and neutrons
• Hydrogen atom


  Has a very small mass
• Uranium atom


  Has a much larger mass
• Relative atomic mass


  How many times heavier an element is compared to a twelfth of the mass of a carbon atom
• We don't actually use hydrogen as the reference, instead we say a carbon atom has a mass of 12
• The masses of other atoms are relative to a twelfth of the mass of a carbon atom
• Important relative atomic masses


  • Carbon: 12
  • Nitrogen: 14
  • Oxygen: 16
  • Hydrogen: 1
• Finding the relative atomic mass

  1. Look it up on the periodic table

  2. The big number is the relative atomic mass
• Relative molecular mass


  The relative mass of a molecule
• Relative formula mass


  The relative mass of an ionic compound
• Relative atomic mass is how heavy an element's atom is compared to a twelfth of the mass of a carbon atom
• Relative molecular mass is for molecules
• Relative formula mass is for ionic compounds
• A mole is a very big number, 6.02 x 10^23
• Mole


  The number of carbon atoms in 12 grams of carbon
• Number of atoms in 1 mole of:

  • Carbon
  • Magnesium
  • Helium
• Calculating moles of atoms

  1. 12 grams of carbon = 1 mole

  2. 120 grams of carbon = 10 moles

  3. 1 kilogram of carbon = 83.3 moles
• Molar mass


  The mass of 1 mole of a substance, equal to its relative atomic mass in grams
• Calculating moles from mass


  Mass / Molar mass = Moles
• Molar masses:


  • Carbon: 12 g/mol
  • Carbon dioxide: 48 g/mol
  • Sodium chloride: 58.5 g/mol
• Calculating atoms from mass


  Mass / Molar mass = Moles

  Moles x 6.02 x 10^23 = Number of atoms
• Chemists prefer to deal with the number of moles rather than the mass
• The mass of 1 mole of a substance is its molar mass
• Calculating moles and mass

  • 1 g of lithium = 0.1 mol

  1 g of iron = 0.02 mol
• Calculating masses in reactions


  1. Balanced chemical equation

  2. Mole ratios

  3. Molar masses

  4. Moles of reactant

  5. Moles of products

  6. Mass of products
• Hydrochloric acid + Sodium hydroxide


  • Neutralization reaction
  • Produces sodium chloride + water
• 100 grams of hydrochloric acid (HCl) reacts with an unknown mass of sodium hydroxide (NaOH)
• Determining mass of sodium hydroxide that completely reacts with 100 grams of hydrochloric acid

  1. Balanced equation

  2. Molar masses

  3. Moles of HCl

  4. Moles of NaOH

  5. Mass of NaCl produced
• The chemical equation is balanced
• Mole ratio


  1 mole HCl reacts with 1 mole NaOH to produce 1 mole NaCl and 1 mole H2O
• Calculating moles of HCl in 100 grams

  1. Molar mass of HCl

  2. Moles of HCl = 100 g / 36.5 g/mol
• Calculating moles of NaOH needed

  Same mole ratio as HCl
• Calculating mass of NaCl produced

  1. Moles of NaCl = Moles of HCl

  2. Molar mass of NaCl
• Calculating moles and mass of H2O produced

  1. Mole ratio from balanced equation

  2. Molar mass of H2O
• Magnesium nitrate decomposes when strongly heated into magnesium oxide, nitrogen dioxide, and oxygen
• Calculating amounts of gases produced from 3.5 grams of magnesium nitrate


  1. Balanced equation

  2. Mole ratios

  3. Molar masses

  4. Moles of reactant

  5. Moles of products

  6. Mass of products
• The chemical equation for the decomposition of magnesium nitrate is balanced
• Mole ratio


  1 mole Mg(NO3)2 produces 1 mole MgO, 2 moles NO2, and 0.5 moles O2
• Calculating moles of Mg(NO3)2 in 3.5 grams

  Molar mass of Mg(NO3)2
• Calculating moles of each product


  Mole ratios from balanced equation