C4 - Chemical Calculations

• Relative formula mass

  The mass of a compound compared to the mass of carbon-12 atoms
• Calculating relative formula mass


  Add up the relative atomic masses of the elements in the compound, in the ratio shown by its formula
• Mole

  One mole of any substance is its relative formula mass, in grams
• Avogadro constant is 6.02 × 10^23 per mole
• Balanced symbol equation


  Tells you the number of moles of substances involved in a chemical reaction
• Calculating masses of reactants and products

  Use balanced symbol equations
• Deducing balanced symbol equations

  From the masses (and hence the ratio of the numbers of moles) of substances involved in a chemical reaction
• Limiting reactant


  The reactant that gets used up first in a reaction, not in excess
• The amounts of product formed in a chemical reaction are determined by the limiting reactant
• Yield

  How much product is made in a chemical reaction
• Percentage yield


  How much product is made compared with the maximum amount that could be made (100%)
• Factors affecting yield

  • Product being left behind in the apparatus
  • Reversible reactions not going to completion
  • Some reactants may produce unexpected reactions
  • Losses in separating the products from the reaction mixture
• Atom economy

  Compares the relative formula mass of the desired product with the sum of the relative formula masses of the reactants, usually expressed as a percentage
• It is important to maximise atom economy in industrial processes to conserve the Earth's resources and minimise pollution
• Calculating mass of solute in a volume of solution

  Calculate mass of solute in 1 dm³ (1000 cm³) of solution

  2. Calculate mass of solute in 1 cm³ of solution

  3. Calculate mass of solute in the given volume of solution
• A more concentrated solution has more solute in the same volume of solution than a less concentrated solution
• Titration

  Used to measure accurately what volumes of acid and alkali react together completely
• End point

  The point at which a reaction between an acid and an alkali is complete
• Acid/base indicator


  Used to show the end point of the reaction between an acid and an alkali
• Calculating concentration in mol/dm³

  Calculate mass of solute in 1 cm³ of solution

  2. Calculate mass of solute in 1000 cm³ of solution

  3. Convert mass (g) to moles
• Concentration is the number of moles of solute in 1 dm³ or 1000 cm³ of its solution
• Using titration to find unknown concentration

  Know the accurate concentration of one solution

  2. Establish the end point

  3. Use the balanced equation to get the number of moles in a certain volume of solution

  4. Multiply up to get the concentration in mol/dm³ (can convert to g/dm³ if needed)
• Percentage yield =


  Actual mass of product produced

  ------------------------------- ×100

  Maximum theoretical mass of

  product possible
• Percentage atom economy =

  relative formula mass of the

  desired product from equation

  ---------------------------- ×100

  sum of the relative formula

  masses of the reactants from

  equation
• Concentration (g/dm³) =


  Amount of solute (g)

  ----------------------

  volume of solution (dm³)
• Number of moles of gas =


  Volume of gas (dm³)

  -------------------

  24dm³
• A certain volume of gas always contains the same number of gas molecukes under the same conditions
• The volume of 1 mole of any gas at room temperature and pr3ssure is 24dm³ (24,000cm³)
• You can use the molar gas volume and balanced symbol equations to calculate volumes of gaseous reactants or products