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.