C3 Quantitative Chemistry

• Mr = Relative formula mass
• Mr = the sum of all relative atomic masses of a molecule
• The law of conservation of mass

  • No matter is lost or gained during a chemical reaction
  • Mass of reactants = mass of products
• Why are chemical equations balanced?

  To show conservation of mass
• Finding the % mass of an element in a compound

  • Ar x number of atoms of the element / Mr of the compound
  • x 100
• Mole = the amount of substance that contains an Avogadro number of particles
• Avogadro’s constant

  $6.02 × 10^23$
• Mass = Mr x Mol
• Mol = Mass / Mr
• Mr = Mass / Mol
• How to prove that mass is conserved in chemical reactions

  • Add all of the Ar on each side to get Mr
  • Mr should be equal if mass is conserved
• Thermal decomposition = breakdown of a product using heat into multiple products
• Gas as a reactant in reactions

  1. The mass of the product seems to increase
  2. because one of the reactants is a gas
  3. and gas can’t be measured
  4. The gas reacts to form part of the product so the mass increases
  5. HOWEVER due to conservation of mass the mass doesn’t actually increase
• Gas as a product in a reaction

  1. The mass of the product appears to decrease
  2. because gas could be produced as a produced
  3. and dissipate to the surroundings so the mass decreases
  4. HOWEVER due to conservation of mass the mass is the same
• Ways to identify uncertainty

  1. Look at the resolution of the measuring equipment - plus or minus half the resolution
  2. Look at your results - mean plus or minus half the range of measurements
• Avoiding uncertainty when measuring volume

  1. Measure the bottom of the meniscus
  2. Judge the meniscus at eye level
  3. This reduces parallax error
• Parallax error = error due to the eye position of the observer
• Avoiding uncertainty when measuring with a gas syringe

  1. Seal the gas syringe tightly - gas could escape, inaccurate volume could be recorded
  2. Set to zero before using - avoids zero error
• Reasons for uncertainty

  1. Impure substances as products or reactants
  2. Substances not fully reacted
  3. Losses - powder is blown away/ gas escapes
  4. Sample is wet - adds mass and contaminates solid
• Balancing equation using reacting masses

  $Mass =$$Mr × Mol$
• Big number in equations = how many moles
• Small numbers in equations = how many atoms of each element there are
• Limiting reactant

  • Reactions stop when one of the reactants is completely used up
  • Other reactants are in excess
  • Product amount formed is directly proportional to the limiting reactant
• Limiting reactant = the reactant that is used up first in a reaction
• 1 mol of gas occupies 24 dm^2 at room temp
• Volume of a gas = Mass/Mr x 24
• Concentration= the amount of substance in a given volume
• More solute = more concentrated
• Diatomic molecules = elements that exist as 2 atoms bonded together
• Law of conservation of mass

  • No matter is gained or lost in a reaction
  • So the mass of reactants = mass of products
  • This is the reason why chemical equations must be balanced
• % mass of an element in a compound

  (Ar x number of atoms of that element ) / Mr of compound

  Then x100