Guidelines for Yield Reporting in Lab Reports

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 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!

1. Loss during transfers. Identify specifically the most problematic transfers involved in the procedures.
2. 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?
3. 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.
4. 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.