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


  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?

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

  3. Where did you get the 160 from?

    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
      (2*56) + (3*16)= 160

  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

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

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