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 various entries in your
notebook meant and 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
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 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. What's the difference
between a 79.8 and an 80.0% yield? Nothing!
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 its really close to the experimental
value measured because the main source of error is the
mass measurement. 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.