Lesson 3 Oxygen (O2)

• Method for determining the percentage of oxygen in the air

  1. Set up the apparatus so that there is 100 cm3 of air in one gas syringe and none in the other.
  2. Heat the copper metal strongly using the blue Bunsen flame and pass the air in the syringe slowly back and forth over the heated metal.
  3. Note the volume of air each time you pass the air over the metal and continue until the volume no longer decreases.
  4. Leave the apparatus to cool for a few minutes and read of the final volume of gas left in the syringe.
• How to calculate the results of the percentage of oxygen in the air.

  (i)  VOLUME OF AIR AT START = 100cm3

  (ii)  VOLUME OF AIR AT END = 80 cm3

  (iii)  THEREFORE VOLUME OF OXYGEN REMOVED = 20 cm3

  iv)    THEREFORE % OXYGEN =   20/100   x 100   = 20%

             

  The 80cm3 of gases left in the syringes are called the RESIDUAL GASES.
• VOLUME OF AIR AT START = 60cm3

  FINAL VOLUME OF AIR = 50cm3

  THEREFORE VOLUME OF OXYGEN = 10 cm3

  % OF OXYGEN IN AIR =  10/60   X 100  = 16.67%

          

  Why is this value less than expected?

  Not all of the oxygen has been removed.
• VOLUME OF AIR AT START = 60cm3

  FINAL VOLUME OF AIR = 50cm3

  THEREFORE VOLUME OF OXYGEN = 10 cm3

  What error might the student have made during the procedure?

  The apparatus was not heated until the volume stopped going down
• How does copper change its appearance during the experiment?


  FROM: pinkish-red/brown   TO: black
• Write a word equation for the reaction

  copper     + oxygen→ copper(II) oxide
• Is this a physical or chemical change?

  A chemical change (synthesis) as a new chemical is formed.
•  How does the copper change in mass? 

  It increases in mass.
• If the air was not heated to constant volume would we get a result greater or less than 20% for the % of oxygen in air?

  Less - as not all of the oxygen would have been removed.
• Give a use for each of the following gases;

  Carbon dioxide: Fire extinguishers.

  Argon: To fill light bulbs.