quantitive chemistry

• Law of conservation of mass:

  • No atoms are lost or made during a chemical reaction
  • Mass of the products equals the mass of the reactants
• Balanced equation of magnesium reacting with hydrochloric acid:

  Mg (s) + 2 HCl (aq) → MgCl2 (aq) + H2 (g)
• Define relative atomic mass (RAM) and relative formula mass (RFM):

  • RAM: Average mass of atoms in an element considering masses and abundance of isotopes, relative to 12C
  • RFM: Sum of RAMs of all atoms in the formula
• Relative formula mass of:

  1. CaF2 - 78
  2. C6H12O6 - 180
• Explanation for why carbon dioxide and water escape from the test tube:

  • They are both gases
• Mean mass of magnesium produced:

  (3.3 + 3.5 + 3.2) / 3 = 3.3

  Suggestion to increase precision:

  • Measure to more decimal places
  • Use a more sensitive balance/apparatus
• Avogadro's constant:

  • Number of atoms, molecules, or ions in a mole of a substance
  • Value is 6.02 x 10^23
• Formula linking mass, molecular mass, and moles:

  • Mass = Mr x Moles
• Mass of 20 moles of calcium carbonate, CaCO3:

  • Mass = Mr x Moles
  • Mr = 100
  • 100 x 20 = 2000 g
• Amount of carbon dioxide in moles in 0.32 g of CO2:

  • Moles = Mass / Mr
  • 0.32 / 44 = 0.007
• Mass of nitrogen needed to form 6.8 tonnes of ammonia:

  Step 1: Number of moles of ammonia = 400000

  Step 2: Number of moles of nitrogen = 200000

  Step 3: Mass of nitrogen = 5.6 tonnes
• Limiting reactant in a chemical reaction:

  • Involves two reactants
  • One reactant is used in excess to ensure complete reaction of the other
  • The reactant that is completely used up is the limiting reactant
• Two formulae that link concentration, mass, and volume together:

  Concentration (g per dm3) = Mass (g)/Volume (dm3)
• Concentration (g per dm3) = Mass (g)/Volume (dm3)
• Concentration (mol per dm3) = nr of moles/volume (dm3)
• 1 mole of a gas at room temperature and pressure occupies 24 dm3
• A technique for finding the concentration of a solution by reacting a known volume of this solution with a solution of known concentration
• How to conduct a titration:

  Rinse the pipette with a solution of unknown concentration and measure out the known volume of this solution

  Add an indicator (a substance that changes colour at the end of titration)

  Rinse the burette with a solution of known concentration, discard the liquid, and gradually add the solution of known concentration

  When the indicator changes colour (at the end point), record the volume added
• Why is it not always possible to obtain the theoretical amount of product in a chemical reaction?

  The reaction may not go to completion because it is reversible

  Some of the product may be lost when it is separated from the reaction mixture

  Some of the reactants may react in ways different to the expected reaction (side reactions may occur)
• % Yield = Actual mass of a product x 100% / Maximum theoretical mass of product
• Calculate the percentage yield of NH3 if 40.5 g NH3 is produced from 20.0 mol H2 and excess N2

  Step 1 - Write the balanced equation N2 + 3 H2 → 2 NH3

  Step 2 - Calculate the theoretical amount of NH3; Moles NH3 = 20/1.5 = 13.3 moles; 13.3 x 17 (Mr of NH3) = 227

  Step 3 - Calculate percentage yield of NH3; 40.5/227 x 100 = 17.8%
• Atom economy: A measure of the amount of starting materials that end up as useful products. It is a ratio of the relative formula mass of the desired product to the sum of relative formula masses of reactants