Penny Lab Conclusion

A lot of you did your best on the Penny Lab conclusion, but your best efforts were not quite aligned with what is best practice.  I’d like to put this up to provide you with guidance, to make the rest of your lab conclusions first rate.

First of all, the conclusion is to be a “stand alone” portion of the lab, in that it explains what you did, how you did, and why you were imperfect.  I am not your audience really.  Imagine you are writing for a science journal, and strangers who didn’t give you directions are reading your report.  They should be able to read it, and reproduce the experiment from your description.  They don’t need (nor do I) such minute detail along the lines of: I started with 56.7 mL of water and when I put in 9 old pennies the volume was 59.7 mL and when I subtract 59.7 mL by 56.7 mL, I get 3.0 mL volume.  TMI!

Tell what you actually did, but not every single step.  Tell what you calculated, which is NOT what you measured.  State your percent error and a reasonable excuse.  You can’t blame it on doing math wrong, you’re not allowed to do math wrong.  End with something scientific.

Only say what you mean, and make sure you mean what you do say.  We did not “compare” the old penny and new penny density.  We didn’t do that.

Speak clearly.  Avoid saying “it” because it’s not clear what you mean.  Say things like “I measured the old pennies to have a measured density of 8.6 g/mL” instead of “they had…”  Be really clear when you write.

Here’s a good template for your conclusion for your penny lab:


I measured 30 old (pre-1982) US pennies mass and volume and plotted that data on a graph.  I drew the best fit line for my data, and drew the best fit line for the points.  Because I plotted mass as a function of volume, the slope of the line equals the  “average” density of the older pennies.  I measured it to be ______ g/mL, but since the pennies are copper (density 8.96 g/mL) my percent error was _____% (must be positive or negative).  My error was most likely due to misreading the graduated cylinder.

I did the same for 30 new pennies (post 1982) and did the same math.  My slope for that best fit line was _____ g/mL, which indicates the metal inside these copper coated pennies was _______ (what ever metal element fits your slope).  Since the metal is really zinc, my percent error for this measurement was ___ %.

I learned that density is a constant, and with care, I can measure and discover what an unknown might be by checking my measure with a chemistry reference table.

I love chem, the end.


That’s all it takes.  Not too wordy.  We all know your data is all attached so we can see if your numbers truly work out to what you state that they do.

A few more important facts:  Never ever round away significant figures.  If your scale says 103.41 grams, do not round it to 103 grams or even 103.4 grams.  We paid a lot of money to measure exactly to the 100th of a gram.

Only round to the proper SF at the very last moment, not in the middle of the math problem.

Pay attention to details like, known density from Table S have UNLIMITED SF, but your measures do not.

Read the directions, and the WHOLE LAB before you start working.  Don’t hand in work with question marks, those are just plain wrong (ex:  -1)

Write clearly.  Stay off of the internet.  Call your teacher if you need help and never make stuff up.  Don’t mix up the data, and if you do you’ll get huge percent errors.  If your percent error exceeds 10% (positive or negative) call your teacher quickly).

Think:  if your new penny percent error is way over, and the old penny percent error is way under, maybe you mixed up your data.

Don’t leave blanks, or think I won’t notice you skipped something. Use lots of blank space so I can send you little notes.  Never say “I might have done the math wrong”, that’s just nuts.  Titles of graphs MUST describe.  “Old Pennies” is not a title, nor does it describe what your graph shows.

If you lost even one point to significant figures errors you should come in for some tutoring right away.  This type of problem never goes away by itself and it will ingrain in you.  Fix this, I can help.

Never quit on yourself, I won’t.  Fight back (academically) and don’t be afraid to question your grade.  Even if I mistakenly take just one point from you, I don’t want to do that, I don’t want to penalize you ever by mistake.  If you get points from me in error, don’t bring this to my attention, that would be dopey.

it’s a long year, you will refine yourselves until June.  It’s called learning, and some lessons take time.  Never cry, just ask for help, I’ve got your back.  It’s not you against me, it’s me helping you be the best you that you can be.