Howdy,
One of the Chem Lab supervisors here. We were really happy (and surprised) about how well everyone did, and it was great seeing all of you!
In
this folder, you can find the exam and answer key. Also included are some data figures, the citric/carbonic acid score calculator, a feedback form, and some “Side B” questions for post-tournament practice. (Here’s a
direct link to the feedback form: all feedback will be read and appreciated!)
Lab Portion
Last year, the lab was the last question and worth a very small portion of the exam. Our reasoning was that since it was meant to be such a fast lab, it shouldn’t be worth too many points. As a result, very few teams attempted the lab last year, and I pipetted out 80 vials of HCl for nothing.
This year, the lab portion was worth 70/200 (35%) of the entire exam. While it’s technically possible to place, it would be downright impossible to win without it. It’s also a really long lab. If you convert points to time, this lab is projected to take 35 minutes. It probably takes a bit longer than that, but you get the idea.
The high score on the lab question was
68.77 (98.2%). A handful of teams got perfect or near-perfect scores on citric/carbonic acid determination.
A few teams got upset that we didn’t provide any glassware, but as stated in 2b on the rules, teams are allowed to bring anything listed on the Recommended Lab Equipment List. Those that don’t bring these things will be at a disadvantage.
Written Portion
Original drafts of the written portion were shortened significantly, with the intent of allowing students to attempt every question. Performance was generally good, with a high score of
93.5 (71.9%).
Specific Questions and Funny Things
Someone claimed that “aqueous solutions do not always contain water.” Another team wrote a treatise about phlogiston. It was a good time.
A lot of competitors asked what our policy on sig figs was. We told everyone some variation of “Try your best”, because the real answer (at least for me) was, “If it’s not 1 sig fig, or 8 sig figs, I’ll probably take it.” But for real, y'all should learn your sig figs, because some Chem Lab answer keys specifically award points for correct sig figs.
#2: It was surprising how few people finished the unit conversions. It’s time consuming, but it’s also easy-ish if you know what the various concentration units are. If you can’t reliably do the other questions, these problems offer great payoff.
On 2.4, a lot of people got baited by“organic and biochemists believe this, so there must be some truth to it!” Two life lessons: biochemists don’t actually know chemistry, and always question appeals to ethos (throwback to AP Lang?). Just because a biochemist says the pKa of water is 15.7 doesn’t mean it’s true. It’s quite common for phony people to cite fancy-sounding credentials in an attempt to deny global warming or whatnot, so it’s important to think critically about claims and titles.
#5 is the classical titration problem. To my surprise, almost no one got the entirety of 5.5 correct. All the volumes of NaOH were selected to be “significant”: i isn’t a buffer (standard Ka = x^2/[concentration]), ii is an endpoint/equivalence point, and iii is a halfway point. The point of 5.4 was to calculate a “vol needed to reach endpoint” value so that it was easier to see that iii was a halfway point. Life hack: in buffer questions, always do a cursory check for endpoints/halfway points. When it comes to halfway points, half (no pun intended) of the work has already been done for you.
6.4 Many teams had some idea of why benzene and toluene form an ideal solution, but few of them took the time to explain why other pairs such as HCl/H2O don't. We were super impressed by how many teams cited "pi stacking" as an IMF shared by toluene and benzene.
Overall high score was
155.67/200 (78.5%). We were extremely pleased by the difficulty and length of the exam; it seemed just-right to us, and there weren't too many ties to break.
Edit: mandatory reminder that aqueous solutions as a topic
effing suck. Physical properties are arguably too open-ended, but it's the topic I also feel the most comfortable writing for since there's so much out there.
