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.
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". 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.) Bottom line: "Lost yield" does not equal
"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.