quantitative chemistry

• What is the law of conservation of mass?

  The law of conservation of mass states that no atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants.
• Write a balanced equation of magnesium reacting with hydrochloric acid


  Mg + 2 HCl -> MgCl^2 + h^2
• Define relative atomic mass

  Average mass of atoms in an element taking into account masses and abundance of its isotopes, relative to 12C.

  
  

  Relative atomic mass = Ar
• Define relative formula mass

  The sum of the relative atomic masses of the atoms in the formula
• How to increase accuracy of experiments measuring mass

  Measure to more decimal places

  Use a more sensitive balance/apparatus
• What is the formula that links mass, molecular mass and moles together

  Mass = Mr x Moles
• What is the mass of 20 moles of calcium carbonate, CaCO3?

  Mass = Mr x Moles

  Mr = 100

  100 x 20 = 2000g
• Calculate the amount of Carbon Dioxide moles in 0.32 g of carbon dioxide

  Moles = Mass x Mr

  0.32 / 44 = 0.007
• Calculate the mass of nitrogen needed to form 6.8 tonnes of ammonia

  Step 1- Work out the number of moles of ammonia (Mr of ammonia = 17)

  6800000 / 17 = 400000 moles of ammonia

  Step 2 - Use the balanced equation and number of moles of ammonia to work out the number of moles of nitrogen

  The ratio of nitrogen to ammonia is 1:2

  Therefore the number of moles of nitrogen is 4000000/2 = 2000000

  Step 3 - Work out the mass of nitrogen (Mr is N^2 is 28)

  2000000 x 28 = 5600000 = 5.6 tonnes
• State what we mean by a limiting reactant in a chemical reaction

  In a chemical involving two reactants , it is common to use an excess of one of the reactants to ensure that all of the other reactant is used. The reactant because it limits the amount of products.
• What is titration?

  A technique for finding the concentration of a solution by reacting a known volume of this solution with a solution of known concentration
• How do you conduct a titration

  a) Rinse the pipette with a solution of unknown concentration. Use the pipette to measure out the known volume of this solution.

  
  

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

  
  

  c) Rinse the burette with a solution of known concentration. Discard the liquid.

  Use a burette to gradually add the solution of a known concentration.

  
  

  d) When indicator changes colour (at the end point), the volume added is

  recorded.

  
  

  e) It is important to get concordant volume results - they have to lie close to each

  other.

  
  

  f) Suitable calculations are performed to find the concentration.