3. Quantitative chenistry

• Relative Formula Mass


  The relative atomic masses of all the atoms in the molecular formula added together
• Calculating relative formula mass is straightforward, but things can get more confusing when working out the percentage compositions of compounds
• Compound


  Has a Relative Formula Mass, M, which is the relative atomic masses of all the atoms in the molecular formula added together
• Finding the relative formula mass of MgCl

  1. Look up the relative atomic masses of Mg (24) and Cl (35.5)

  2. Add the relative atomic masses (24 + 35.5 x 2 = 95)
• Percentage mass of an element in a compound

  (Relative atomic mass x number of atoms of that element) / Relative formula mass of the compound x 100
• Calculating the mass of iron chloride (FeCl₂) needed to provide the iron ions in 50g of a 20% iron mixture

  1. Find the mass of iron in the 50g mixture

  2. Calculate the percentage mass of iron in iron chloride

  3. Calculate the mass of iron chloride needed to provide 10g of iron
• Mole


  The amount of a substance that contains 6.023 x 10^23 particles (atoms, molecules, ions etc.)
• The mass of one mole of a substance is equal to its relative formula mass (M) in grams
• Number of moles


  Mass in g / Relative formula mass (M)
• In a chemical reaction, the total mass of reactants equals the total mass of products (conservation of mass)
• If the mass seems to change in an experiment

  It's usually because a gas is involved - either being taken in from the air or escaping from the reaction vessel
• In a balanced chemical equation, the total relative formula mass of the reactants equals the total relative formula mass of the products
• Moles can be used to calculate masses in chemical reactions
• Balancing a chemical equation using masses

  1. Find the mass of each substance

  2. Divide the mass of each substance by its relative formula mass to find the number of moles

  3. Use the mole ratios to balance the equation
• Show that mass is conserved in the reaction: H2SO4 + 2NaOH → Na2SO4 + 2H2O

  1. Write the balanced equation

  2. Calculate the relative formula masses

  3. Calculate the number of moles of each substance

  4. Use the balanced equation to work out the number of moles of product formed

  5. Calculate the mass of the product
• A(C)=12, A(O)=16, A(Na)=23, A(S)=32
• This is the moment where the 'number of moles = mass / molar mass' equation from page 42 is used
• Moles


  The unit used to measure the amount of a substance
• Using moles to calculate masses in reactions

  1. Write the balanced equation

  2. Calculate the relative formula masses

  3. Calculate the number of moles of each substance

  4. Use the balanced equation to work out the number of moles of product formed

  5. Calculate the mass of the product
• Balanced equations tell you the mole ratios of the reactants and products
• Reactions don't go on forever, they stop when one of the reactants is used up
• Limiting reactant


  The reactant that is completely used up in a reaction, limiting the amount of product formed
• The amount of product formed is proportional to the amount of limiting reactant
• Calculating the mass of a product formed

  1. Write the balanced equation

  2. Calculate the relative formula masses

  3. Calculate the number of moles of the known substance

  4. Use the balanced equation to work out the number of moles of product formed

  5. Calculate the mass of the product
• Theoretical yield


  The maximum amount of product that can be formed in a reaction, calculated using the balanced equation
• In practice, the actual yield is less than the theoretical yield
• One mole of any gas occupies 24 dm³ at room temperature and pressure
• Calculating the volume of a gas

  1. Find the mass of the gas

  2. Find the molar mass of the gas

  3. Use the formula: volume = mass / molar mass * 24
• Calculating the volume of a gas in a reaction

  1. Write the balanced equation

  2. Use the mole ratios to find the volume of one gas from the volume of another
• Concentration


  A measure of how much solute is dissolved in a given volume of solution
• Calculating the concentration of a solution in g/dm³

  1. Find the mass of solute

  2. Find the volume of solution in dm³

  3. Use the formula: concentration = mass of solute / volume of solution
• Calculating the concentration of a solution in mol/dm³

  1. Find the number of moles of solute

  2. Find the volume of solution in dm³

  3. Use the formula: concentration = number of moles of solute / volume of solution
• Titrations are experiments that find the volumes needed for two solutions to react completely
• Finding the concentration of a solution using a titration

  1. Use the balanced equation to find the moles of one solution

  2. Use the volume of that solution to calculate its concentration
• Concentration


  The amount of a substance dissolved in a given volume of solution, usually expressed in mol/dm³
• Finding the concentration in mol/dm³

  1. Determine the number of moles of the known substance

  2. Use the reaction equation to determine the number of moles of the unknown substance

  3. Calculate the concentration of the unknown substance using the number of moles and volume
• Concentration formula


  Concentration = number of moles / volume
• Steps to find concentration of unknown substance

  1. Calculate the moles of the known substance (NaOH)

  2. Use the reaction equation to determine the moles of the unknown substance (H₂SO₄)

  3. Calculate the concentration of the unknown substance using the moles and volume
• Converting concentration from mol/dm³ to g/dm³

  • 1. Find the relative formula mass of the substance
  • 2. Multiply the concentration in mol/dm³ by the relative formula mass to get the concentration in g/dm³
• Titration experiments involve some uncertainty in measurements, so the average (mean) of repeated measurements is calculated, and the range can be used to estimate the uncertainty