Quantitative Chemistry

• Whenever you are measuring something there is going to be a degree of uncertainty whether it's a burette, a measuring cylinder or a beaker
• You need to look for the bottom of the meniscus when measuring as there is a difference between where it looks at the top and where it is at the bottom
• You can never say the measurement is accurate because it might be a quarter or three quarters of the way to the next line
• Concentration


  Amount divided by volume, measured in moles per decimeter cubed
• Balancing an equation


  1. List reactants and products

  2. Circle compounds

  3. Check numbers of each element

  4. Adjust coefficients to balance
• Relative formula mass (Mr)


  Sum of the relative atomic masses of the atoms in a molecule
• Mole


  Unit for the amount of a substance, equal to 6.02 x 10^23 atoms, ions or molecules
• Calculating moles


  Moles = Mass / Mr
• Balancing an equation for hydrogen peroxide decomposition

  1. Write unbalanced equation

  2. Balance by adjusting coefficients

  3. Equation: 2H2O2 → 2H2O + O2
• Calculating mass of oxygen produced

  1. Use balanced equation and ratio of masses

  2. Oxygen mass = (32/68) x 40.8 = 19.2g
• Limiting reactant


  The reactant that is used up first, determines the amount of products formed
• Percentage yield


  Actual yield / Theoretical yield x 100
• Titration procedure


  1. Use pipette to add known volume of one solution

  2. Add indicator

  3. Slowly add second solution from burette until colour change

  4. Record final burette reading

  5. Repeat for consistent results
• Calculating titration results


  1. Find moles of acid from volume and concentration

  2. Moles of acid = moles of alkali due to neutralisation

  3. Use moles of alkali to calculate concentration
• Gas volume


  1 mole of gas occupies 24 dm^3 at room temperature and pressure
• There is a lot of maths involved in quantitative chemistry, including balancing equations, calculating masses, moles, concentrations and gas volumes
• The total mass of all substances is conserved in a chemical reaction
• The atoms that go in must come out, so we must balance equations
• Relative atomic mass


  The mass of an atom relative to the mass of a carbon-12 atom
• Relative formula mass


  The sum of the relative atomic masses of the atoms in a compound
• CO2 has a relative formula mass of 44 (12 + 2 x 16)
• Reactions that produce a gas product can result in a seeming decrease in mass of the reactants
• Mole


  A specific number of atoms or molecules, used to compare amounts of substances
• If you have as many grams of a substance as its relative atomic or formula mass, you have one mole
• Moles


  Calculated as g / relative atomic/formula mass
• Balancing a chemical equation

  Determine the number of moles of each reactant and product needed to balance the equation
• Calculating mass of product from mass of reactant

  Mass -> Moles -> Moles -> Mass
• The stoichiometry of a reaction is the ratio of moles of one substance to another
• One mole of any gas takes up a volume of 24 dm^3 at room temperature and pressure
• You can convert between moles and volume by multiplying or dividing by 24
• Percentage yield


  The amount of product actually made compared to the theoretical maximum
• Atom economy


  The efficiency of a reaction in terms of the mass of desired product compared to the total mass of reactants
• Carbon dioxide is an incredibly important gas necessary for life to thrive