Sunday, 21 April 2013

1.26 calculate percentage yield

Percentage yield is: (actual yield / theoretical yield) x 100

Theoretical yield is what you expect to get, if a reaction doesn't finish you may end up with a lower yield than the expected theoretical yeild.

To work out theoretical yield it is important to understand that an equation gives you a ratio of moles, e.g.
Fe2O3 > 2Fe
tells us that every one mole of iron oxide makes two moles of iron. In this equation the weight of Fe will be your yield.

If we are told the weight of the Fe2O3 is 100g we can easily work out the theoretical yield:
Work out the moles of Fe2O3 by doing the weight (g) divided by the atomic mass: 100/160= 0.625Mol
We know that for every one mole of Fe2O3 there are two of Fe so we do: 0.625 x 2= 1.25 Mol
Now we have moles of Fe we can work out weight by Mol x Ar: 1.25 x 56= 70g

If you in fact got 62g of Fe you'd do (62/80)x100= 77.5%

Inline images 2

12 comments:

  1. "Now we have moles of Fe we can work out weight by Mol x Ar: 1.25 x 56= 70g"
    Shouldn't it be Mol x Mr, as it is is 2Fe? So, it would be 1.25 x 112 = 140g?

    ReplyDelete
  2. Replies
    1. Ar is relative atomic mass
      Mr relative molecular mass

      Delete
  3. Where did you get the 160 from?

    ReplyDelete
    Replies
    1. 160 is the atomic mass of the molecule, this is worked out by adding together the individual atomic masses of all the particles, so in this case:
      Fe= 56
      O=16
      (2*56) + (3*16)= 160

      Delete
  4. You screwed this question up at the end the actual yield I.e 82g should be lower than the theoretical yield as nothing is able to have conversion rates of 100% and therefore the next part is wrong it should be 82/70 which makes no sense as the yield is greater than 100% as you got the actual yield wrong

    ReplyDelete
  5. someone help, why is it 62 over 80? and not 100?

    ReplyDelete
  6. If you got 70g from expected Fe, and 62 for actual Fe, wouldn't it be 62/70 = 0.8857 = 88.57% ?

    ReplyDelete
  7. Calculating Theoretical yield

    Theoretical yield is actually the mass of product you will get (in calculations not real life). It can only be specific compound not more then one.

    To find theoretical yield you first have to know the amount of substances used.

    For example

    2h2o + ar3 = 4hg5

    this means that 2 moles of h2o produces 4 moles of hg5
    and also
    1 mole of ar3 produces 4 moles of hg5

    now one of the reactant's mass\weight will be given

    for example h20 is 20 grams

    so 2 moles of h20 is 2x20 grams = 40 grams

    so now lets find the mass\weight of product(hg5)(Theoretical yield )

    so first

    convert the grams into moles of h20

    now we need RFM (Relative Formula Mass) of the substance

    for example h20 has RFM of 10 (it will also be given)

    mass of one mole is RFM

    number of moles = mass of substance/mass of one mole of the substance

    so 40 /10 = 4

    so 4 moles of h20 is used in this reaction

    so if 4 moles of h20 is used then we will get 8 moles of hg5

    because

    2h2o + ar3 = 4hg5

    this means that 2 moles of h2o produces 4 moles of hg5
    so

    4h2o + 2ar3 = 8hg5

    this means 4 moles of h20 is used then we will get 8 moles of hg5

    (same math increased moles as we have more moles of h2o)


    so moles of hg5 is 8 in this reaction
    so now we will find the mass/weight of hg5 by its moles

    so need to figure out the RFM because

    it will also be given so lets take it as 23

    number of moles = mass of substance/mass of one mole of the substance

    mass of one mole is RFM

    so let "mass of substance" be $$

    8 = $$ / 23

    so $$ = 23 x 8

    so thats the theortical of hg5

    ReplyDelete
    Replies
    1. and then find actual yeild by the formula and the actual yield will be given

      Delete