Always remember to present
your yields and percent yields in the Results Tables.
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 never 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 doing the % yield calculation 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 great,
70-80 good, 40-70 fair, 20-40 poor, 0-20 very poor.
(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. Experimental error in a
yield measurement could be the random error inherent
in measuring the masses of the reactants and the
product (+ or - 0.001 g?). Or it could be systematic
error resulting from the presence of impurities still
present in the product (and artificially inflating its
mass). However, 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 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 and try to identify
ways the yield could be increased if the experiment
were repeated. 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.