Unit 3: quantitative chemistry

• 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 measurements
• Mole


  The unit used to measure chemical amounts
• Molar mass


  The mass of one mole of a substance in grams, 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

  Use the triangle or the equation: mass = moles x molar mass
• 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


  Convert the masses in grams to amounts in moles

  2. Convert the 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


  Use the equation: 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