C3 Quantitative Chemistry

• 4.3.1.1 - Conservation of Mass


  • The law of conservation of mass states that during a chemical reaction no atoms are destroyed or created, meaning there will be the same number and types of atoms on each side of a reaction equation
  • No mass is lost or gained
  • 2Li + F2 > 2LiF
• 4.3.1.2 - Relative Formula Mass


  • The relative formula mass (Mr) of a compound is the sum of the relative atomic masses of the atoms in a formula
  • In a balanced reaction the Mr is the same on both sides
  • e.g. find the RFM of MgCl2 when the Ar of Mg is 24 and the Ar of Cl = 35.5.
  • Mr = 24 + (2X35.5) = 95
• Percentage Composition of Compounds


  • % mass = (Total Ar of the element ÷ Mr of the compound) × 100
• 4.3.1.3 - Mass change including a gas


  • When a reactant or product is a gas their mass is not taken into account
  • When a metal(s) reacts with oxygen(g) the mass of oxide produced is greater than the mass of the metal so the overall mass increases
  • If a gas escapes (steam) the total mass will look like it decreased
• 4.3.1.4 Chemical measurements

  • Whenever a measurement is made there is always some uncertainty about the result obtained.
  • There are two ways of estimating uncertainty:
  • by considering the resolution of measuring instruments or from the range of a set of repeat measurements
  • The uncertainty of a measuring instrument is estimated as plus or minus (±) half the smallest scale division. For a thermometer with a mark at every 1.0°C, the uncertainty is ± 0.5°C. This means that if a student reads a value from this thermometer as 24.0°C, they could give the result as 24.0°C ± 0.5°C.
• 4.3.2.1 - Moles 


  • Chemical amounts are measured in moles (mol)
  • The mass of one mole of a substance in grams is numerically equal to it's relative formula mass and contains the same number of stated particles, atoms, molecules or ions as one mole of any other substance
  • Avogadro Constant is the number of atoms, molecules or ions in a mole and = 6.02 X 10^23
  • Carbon has an Ar of 12 so one mole of carbon weighs 12g
• 4.3.2.3 - Using moles to balance equations


  1. Divide the mass of each substance by it's relative formula mass to find the number of moles
  2. Divide the number of moles of each substance by the smallest number of moles in reaction
  3. If any of the numbers aren't whole numbers then multiply all the numbers by the same amount
  4. Write balanced symbol equation by putting these numbers Infront of chemical formulas

  
• 4.3.2.4 - Limiting Reactants

  • In a chemical reaction the reactant which gets compeltely used up first is the limiting reactant as it makes the reaction stop
  • The amount of product formed is directly proportional to the amount of limiting reactant; if you double the amount of limiting reactant the amount of product will double
  • It is common to use a reactant in excess to ensure all the other reactants get used up
• 4.3.2.5 - Concentrations of Solutions

  • Concentration is the amount of chemical dissolved in a given volume of solution
  • The concentration of a solution is measured in mass per given volume of solution
  • Concentration (g/dm^3) = mass (g) / volume (dm^3)
  • 1ml = 1cm^3
  • 1dm^3 = 1000cm^3
  • Mass can be replaced by the number of moles in an equation
• 4.3.3.1 - Percentage yield


  • Percentage yield tells you the overall success of an experiment and compares what you think you should get (theoretical yield) with what you get in practice (actual yield)
  • The amount of product you get is called the yield
  • Yields are always less than 100% because some products get lost and side reactions or reversible ones can occur

  
• 4.3.3.2 - Atom Economy


  • The atom economy is the measure of reactants that end up as useful products
  • The higher the atom economy the more eco friendly the reaction was since there is less waste materials so it is cheaper
  • The more products the lower the atom economy

  
• 4.3.4 - Using concentrations of solutions

  • The concentration of a solution can be measured in mol/dm^3
  • The amount in moles of solute or the mass in grams of solute in a given volume from a solution can be calculated from it's concentration
  • If the volumes of two solutions that react are known and the concentration of one is known the concentration of the other can be calculated
• Essential Formulas to know

  • Moles = concentration X volume
  • Volume = moles / concentration
  • Concentration = moles / volume
  • Mass = moles X relative formula mass
  • Volume of gas = (mass / molar mass) X 24
  • or volume of gas = moles X 24
• 4.3.5 - Amount of substance in relation to gas


  • equal amounts in moles of gas occupy the same volume under the same conditions of temperature and pressure
  • The volume of one mole of any gas at room temperature and pressure (20o C and 1 atmosphere pressure) is 24dm^3
  • volume of gas = moles X 24
  • volume of gas = (mass / molar mass) X 24