quantitative chemistry (c3)

• Law of conservation of mass

  No atoms are lost or made during a chemical reaction so the mass of the products = mass of the reactants
• Balanced chemical equation

  • The numbers of atoms of each element involved are the same on both sides of the equation
• Relative formula mass (Mr)


  Sum of the relative atomic masses of the atoms in the numbers shown in the formula
• In a balanced chemical equation, the sum of Mr of reactants in quantities shown = sum of Mr of products in quantities shown
• If a reaction appears to involve a change in mass

  Check if this is due to a reactant or a product as a gas and its mass has not been taken into account
• When a metal reacts with oxygen, the mass of the metal oxide product is greater than the mass of the metal
• Whenever a measurement is made there is always some uncertainty about the result obtained
• Estimation of uncertainty

  Be prepared to make estimations of uncertainty when making chemical measurements
• Mole


  The unit used to measure chemical amounts
• The mass of one mole of a substance in grams is numerically equal to its relative formula mass
• One mole of a substance contains the same number of the stated particles, atoms, molecules or ions as one mole of any other substance
• Converting between moles and grams

  Mass = moles x RFM
• The number of atoms, molecules or ions in a mole of a given substance is the Avogadro constant: 6.02 x 10^23 per mole
• Balanced symbol equation

  Can be interpreted in terms of moles
• The total moles of one element must be the same on both sides of the equation
• Balancing numbers in a symbol equation

  1. Convert masses in grams to amounts in moles

  2. Convert numbers of moles to simple whole number ratios
• Limiting reactant

  The reactant that is used up / not in excess (since it limits the amount of products)
• If a limiting reactant is used, the amount of product produced is restricted to the amount of the excess reactant that reacts with the limiting one
• Concentration of a solution

  Measured in mass per given volume of solution e.g. grams per dm^3 (g/dm^3)
• Calculating mass of solute in a given volume of a known concentration

  Mass = concentration x volume
• Smaller volume or larger mass of solute


  Gives a higher concentration
• Larger volume or smaller mass of solute


  Gives a lower concentration
• Percentage yield

  Amount of product produced / Maximum amount of product possible x 100
• Reasons why calculated amount of product may not be obtained

  • Reaction may not go to completion because it is reversible
  • Some of the product may be lost when it is separated from the reaction mixture
  • Some of the reactants may react in ways different to the expected reaction
• Yield

  Amount of product obtained
• Calculating theoretical mass of a product

  1. Calculate mol. of reactant by using mol. = mass / molar mass

  2. Use balancing numbers to find mol. of product

  3. Calculate theoretical mass of a product by using mass = mol. x molar mass
• Atom economy

  A measure of the amount of starting materials that end up as useful products
• Atom economy is important for sustainable development and for economic reasons to use reactions with high atom economy
• Possible reasons why a particular reaction pathway is chosen/not chosen

  • Atom economy
  • Yield
  • Rate
  • Equilibrium position
  • Usefulness of by-products
• Concentration of a solution

  Measured in mol. per given volume of solution e.g. mol. per dm3 (mol./dm3)
• Calculating concentration

  1. Mass of solute

  2. Volume of solution

  3. Moles = concentration x volume
• Calculating moles from mass

  Mol. = mass / molar mass
• Calculating concentration from known moles and volume

  1. Moles = concentration x volume

  2. Concentration = moles / volume
• Calculating concentration of unknown solution

  1. Work out moles of known solution

  2. Use mole ratio from equation to find moles of unknown solution

  3. Concentration = moles / volume
• If the volumes of two solutions that react completely are known and the concentration of one solution is known, the concentration of the other solution can be calculated
• Equal amounts (in mol.) of gases

  Occupy the same volume under the same conditions of temperature and pressure (e.g. RTP)
• Volume of 1 mol. of any gas at RTP (room temperature and pressure: 20 degrees C and 1 atmosphere pressure)


  24 dm3
• Equation to calculate volume of gas at RTP

  Volume of gas (dm3) at RTP = Moles x 24
• Using the equation, if the reaction is at RTP, you can calculate moles of a gas produced and then x24 to get volume produced (e.g. if you produce 5 moles of hydrogen, you produce 24 x 5 = 120 dm3)