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Chemistry Exam on Tuesday

• Message 1. 

  Posted by U15108832 (U15108832) on Sunday, 3rd March 2013

  Could someone please give me a list of all the quantitive chemistry formulas we need to know? and a basic summary of each one (i.e the equation needed) Im getting confused!!
  
  Thanks to any help in advance!!
  x

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  Posted by The Bitesize Science Teacher (U14392404) on Sunday, 3rd March 2013

  The list will depend on the exam board, course and paper your test is based on.
  
  This Bitesize section should get you started: www.bbc.co.uk/school...

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  Posted by Lauren (U15635288) on Sunday, 3rd March 2013

  Topic 6
  1. RFM is really simple. You will need the atomic mass number – which is the bigger one. The RFM is just all the RAMs added together. Remember to add up the RAMs for molecules and times everything in a bracket by the number outside the bracket.
  2. Percentage composition by mass is also brilliant and easy.
  Percentage mass OF AN ELEMENT IN A COMPOUND = (atomic mass x number of atoms of that element) / atomic mass of whole compound x 100
  3. ‘Empirical formula’ sounds tons worse than it actually is.
  • List all the elements in the compound
  • Underneath them, write the mass used in the experiment
  • Divide each mass by the atomic mass for that particular element
  • Turn the number you get into a lovely little ratio
  • When the ratio is in the simplest form, it tells you the formula
  • DONE!
  4. Calculating masses in reactions:
  • Write out the balanced equation
  • Work out the atomic mass for the two bits you want
  • Apply this rule: ‘divide to get one, multiply to get all’
  • In practice: What mass of magnesium is produced when 60g of magnesium oxide is burned in air?
  2Mg + O 2  MgO
  2x24 = 48 2 x (24+ 16) = 80
  RULE:
  - 48 g of magnesium gives 80 g of magnesium oxide
  - 1 g of magnesium gives ...
  - 60 g of magnesium gives ...
  • So, you divide 80/48 -> 1.666666
  • Then multiply 60g by 1.666666
  ANSWER: 100g
  5. Percentage Yield = actual yield/theoretical yield x100
  - percentage yield compares actual yield with theoretical yield
  - yield is the mass of product obtained in a reaction
  - the actual yield of a reaction is usually less than the theoretical yield
  6. Percentage yield =
  

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