APPENDIX D:
A PERCENT YIELD CALCULATION METHOD SUMMARY
The calculation of a per cent yield is a very important
part of those labs where a chemical synthesis has been done. In
your report be sure to correctly do the percent yield
calculation (showing the correct significant figures and units)
and present it in a very clear manner. Do not forget the show
the correct units with each measurement as you perform the
calculation. A percent yield calculation is an application of a
weight weight problem which you learned in Fundamentals of
Chemistry.
You must show how the calculations are done in an area that
is separated from text. Do not put data details in paragraph
text. Present data details in tabular form and be sure this
data is near the text that makes reference to it for clarity of
presentation or, if the calculations have been attached to the
end of your report as an appendix, give a clear sentence that
refers the reader to this page. Calculations should be close to
where the final data is presented in your report. Briefly, a
percent yield calculation is an application of a weight-weight
problem in General Chemistry. You calculate the number of moles
(or millimoles) of each reagent. Determine which reagent is the
limiting reagent from the balanced equation. Catalysts are not
usually limiting reagents. Then, determine from the balanced
equation the maximum number of moles of the desired product that
could be obtained and this is your theoretical yield in moles.
Convert this number of moles of product to a theoretical weight
of product. Then, divide the weight of product that you
obtained by the theoretical weight of product you calculated and
multiply by 100 %. Alternatively, you could determine the
actual moles of product obtained by taking the mass of the
product isolated and dividing by its molecular weight. The
percent yield in that case would be the actual moles divided by
the theoretical moles times 100 %. Be sure to identify which
method you use by showing numerical and unit values for all data
in your calculations.
Percent yield calculations will be a
major part of your report grade in those labs where percent
yield data needs to be reported.
SAMPLE CALCULATION:
Below is just a guide to show you how the calculations are done
presented in a manner to help you understand the process. This
is not an acceptable percent yield calculation for your lab
report. The method below was adapted from a reaction of a
diamine with an acid to give a salt which appeared in week 8 of
Organic Chemistry I lab.
The balanced equation showed that 1 mole of diamine reacts with
1 mole of acid to give 1 mole of salt.
You started with 1.20 ml of diamine.
1.14 g of diamine
1.20 ml * .951 g/ml gives
1.14 g of diamine / 114 g/mole of diamine gives .0100 mole of
diamine used.
You also weighed some tartaric acid that varied from group to
group. Say your group weighed .732 g.
The number of moles of acid in the .732 g is .732 g / 150 g/mole
of tartaric acid or 0.00488 mole of tartaric acid.
From the balanced equation, the acid is the limiting reagent and
the theoretical yield of the product would be .00488 moles of
salt.
From the molecular weight of the product, the theoretical weight
of the product is 0.00488 mole * 264 g/mole of salt or 1.29 g.
If you obtained .847 g of product, your percent yield is
(.847 / 1.29 g) * 100% or 65.7 %.
© 2006 STEPHEN ANDERSON AND ROBERT SHINE