C4: chemical calculations

• Relative formula mass

  The mass of a compound calculated from the relative atomic masses of its constituent elements
• Calculating relative formula mass

  1. Look at the relative atomic masses of each element in the compound

  2. Multiply each element's relative atomic mass by the number of that element in the compound

  3. Add up all the masses
• Mole


  A unit of measurement that represents 6.022 x 10^23 particles (atoms, molecules, or ions) of a substance
• Avogadro's constant


  The number of particles in one mole, which is 6.022 x 10^23
• Calculating moles

  Moles = Mass / Relative formula mass
• Balancing chemical equations

  Count the number of each element on both sides

  Adjust the coefficients to balance the equation
• Balanced chemical equation

  Shows the mole ratio of reactants and products
• Mass is conserved in a chemical reaction
• Calculating theoretical yield

  Balance the equation

  Calculate moles of limiting reactant

  Use mole ratio from balanced equation to calculate moles of product

  Convert moles of product to mass using relative formula mass
• Limiting reagent


  The reactant that is completely used up in a chemical reaction, limiting the amount of product that can be formed
• Calculating percentage yield

  Actual yield / Theoretical yield x 100
• Percentage yield is important for chemists because it shows how efficient a reaction is
• It's very unlikely that you would actually ever make 2.2 grams of iron from the reaction of iron oxides with carbon, you'll usually make less because you can never carry out a reaction to its fullest and sometimes you get a bit of transfer loss, it usually results in a value of around 1.9 grams of iron
• Percentage yield

  How much you make compared to the theoretical, calculated by dividing the actual yield by the theoretical yield
• Percentage yields are massively important for chemists because they need to know how successful their reactions are and whether they're losing a lot of their reaction reactants
• Atom economy

  How much of a percent of the starting materials ends up as useful products and not wasted
• The reaction of Mg going to Mg + O2 going to MgO has an atom economy of 100% because all of the reaction is making useful products
• Concentration

  Can be calculated by mass divided by volume or moles divided by volume
• Dilute


  Not much solids dissolved in the liquid
• Concentrated

  Lots of solids dissolved in the liquid
• To make something more dilute, add more liquid. To make something more concentrated, add more solid
• Concentration can be represented in units of grams per cm3, grams per dm3, moles per cm3, or moles per dm3
• Titration


  A technique used to calculate the unknown concentration of an acid or alkali
• Titration procedure

  1. Have an unknown concentration of acid in a conical flask

  2. Have an alkali of known concentration in a burette

  3. Add the alkali to the acid until neutralisation, indicated by an indicator

  4. Use the volumes and concentrations to calculate the unknown acid concentration
• Moles of gas

  1 mole of gas occupies 24 dm3 or 24,000 cm3
• To calculate moles of gas, divide the volume by 24 (for dm3) or 24,000 (for cm3)