Always remember to present your yields and
percent yields both for crude and final products in the
Results Tables
It's best to calculate your percent yield
and enter it in the lab notebook before leaving the lab.
The longer you wait between finishing up the lab work
and starting to analyze the results, the more likely you
are to forget what specific entries in your notebook
mean and to confuse data. Also, this policy will allow
you to catch mistakes that you have made in your
measurements before it is too late.
Base the percent yield on the theoretical
yield. (Not on the "expected yield". Please note that
the theoretical yield is never the "expected yield" as
we seldom expect 100% yield in a chemical reaction.).
Calculate the theoretical yield based on the moles of
the limiting reactant actually used. (The amount
actually used usually is slightly different from that
specified in the plan so you should redo the the
theoretical yield based on actual amounts when writing
the report for an experiment.)
Report percent yield to the nearest
percent only. Most often the figures past the
decimal place are not significant, and, even if they
are, no one is interested in what fraction of a percent
above the nearest whole number you obtained. Think of
percent yield as a grade for the experiment: 90 is
excellent, 70-80 very good, 50-70 good, 40-50
acceptable, 20-40 poor, 5-20 very poor, etc. (Please realize that the above definitions of
what constitutes good, fair, poor, etc. yields
are arbitrary and that other factors play a role. For
example, for the first attempt at a reaction by a
novice chemist a 50% yield may be considered very
good.)
Discuss reasons for lost yield. When
discussing the yield in the Results and Discussion you
should always explore reasons for loss of yield. Please
realize that this is not an "error analysis".
Experimental errors are factors that affect the
certainty of measurements. The most significant
experimental error in a yield measurement usually is the
random error in measuring the masses of the reactants
and the product (+ or - 0.001 g?). People often write
statements to the effect of, "we obtained a 45% yield,
meaning we had 55% error in the experiment". This is not
valid. The percent error in a percent yield is not
(exptl yield - theor yield)/theor yield x 100. Rather,
it is (exptl yield - true yield)/true yield x 100. The
true yield we don't really know for sure, but usually it
is close to the experimental value because the main
source of error is the mass measurement. (An exception
would be if there was an appreciable amount of solvent
still present in the product.) The
bottom line is "Lost yield" does not equate to
"experimental error".
Your aim in discussing reasons for lost
yield is to identify some likely places where yield was
lost so that suggestions for how to improve the yield
can be made. Do not write things like, "yield might have
been lost when....." or "we may have not cooled it down
long enough". Again, we don't want a laundry list of
things that could have gone wrong, we want to identify
likely reasons for lost yield. I suggest you go through
the following check list when trying to identify reasons
behind yield loss. You can list them in your report, but
as you list each one if you can come up with reasons why
it can be ruled out as a significant reason then state
the reason and rule it out!
- Loss during transfers. Identify
specifically the most problematic transfers involved
in the procedures.
- Loss due to reaction inefficiency. Were
there side reactions that wasted the limiting
reactant? Was it a slow reaction, for which a longer
reaction period was needed to achieve complete
conversion of the limiting reactant? Was there an
unfavorable equilibrium constant involved?
- Unavoidable losses during work up. Examples:
In vacuum filtration steps usually the solid being
filtered is slightly soluble in the solvent being
used and so some of the solid stays dissolved in the
liquid filtrate. Similar loss occurs during
extraction steps if the desired compound has any
water solubility (it dissolves into the aqueous
layer). Distillation steps always involve loss due
to the fact that not all material can be expected to
distill out ("hold-up volume"). Drying over sodium
sulfate usually involves a decantation step that
unavoidably leaves behind some of the solution with
the drying agent.
- Procedural mistakes, poor technique, or lab
accidents. Of course, these can play a role,
but this discussion should not be about beating
yourself up or finding blame. If there was a mistake
then, by all means, do note it and it is certainly
fair to note that you are beginners and with
practice you would probably be able to achieve a
better percent yield. Do not use having made a
mistake or accident as an out that gets you out of
discussing more weighty reasons for lost yield.
Also, do not refer to any of these as "human error",
which is a euphemism best left for writers in other
genres to use.