The easiest and the toughest questions in 2017 CCC

Chemistry competitions can help students build their confidence and skills in chemistry and critical thinking. The Chemical Institute of Canada hopes that chemistry teachers across Canada will support their students in enrolling in the Canadian Chemistry Contest (CCC) and/or the Canadian Chemistry Olympiad contest (CCO). Top students on the CCC can win provincial and national recognition and prizes of $400-850. Top scorers for the CCO may qualify for regional chemistry training camps and a chance to represent Canada at the International Chemistry Olympiad which will be in Czech Republic and Slovakia in July 2018. 

The CCC and the CCO will be written on Monday, April 16, 2018. Students can sign up through their teachers. Teachers must complete the registration form^1a  by March 21, 2018. There is no cost associated with registering for the CCC or CCO. While the CCO requires some advance preparation, the CCC material is accessible to all students in a senior level high school chemistry course. The CCC consists of 25 multiple choice questions written in 60 minutes and 2 written essays written in 90 minutes from a choice of three topics. Teachers and students can access the CCC multiple choice exams, answers and essay topics^1b from 2007-2017 and use them as classroom resources and student preparation tools for the 2018 CCC. This article will focus on an analysis of the easiest and the hardest questions on the 2017 CCC as determined statistically by the difficulty index. The average score on the 2017 was 13.2 out of 25 on the multiple choice exam. 

Easiest question on the CCC 
(Difficulty index 1) 

The question students found easiest on the exam introduced the concept of molality, which may not have been familiar. However, all the information needed to calculate molality was defined in the question and 78% of students were able to read the information and correctly calculate the answer. The discriminating index of this question, which compares how well students scoring in the top 27% on the test answered the question compared to those who scored in the bottom 27% was 0.48, which indicates the question, although easy, is an excellent discriminator.² 

Question

The molality (m) of a solution is defined as the number of moles of solute per kilogram of solvent. Lauryl alcohol (C₁₂H₂₆O) is prepared from coconut oil and is used to make sodium lauryl sulfate, a synthetic detergent. What is the molality of a solution of 17.1 g lauryl alcohol dissolved in 3.21 moles of ethanol (C₂H₆O)

A)  0.310 m

B)  0.621 m*

C)  0.842 m

D) 1.41 m

E)  2.52 m

The students did not seem to have much trouble using the chemical formulae to calculate the molar mass of lauryl alcohol and ethanol and then calculate the mass of ethanol and the number of moles of lauryl alcohol.

Hardest question on the CCC 
(Difficulty index 4) 

The hardest question for students involved a combination of stoichiometry and the gas laws. Only 23% of students writing the CCC submitted a correct answer. However, the discriminating index of the question was quite good at 0.35.

Question

A closed 600.0 mL flask contains solid mercuric oxide and air initially at 21.0 ⁰C and 101.3 kPa. When heated, mercuric oxide decomposes completely according to the reaction:

2 HgO(s) → 2 Hg(s) + O₂(g) 

After heating, the flask is at a temperature of 75.2 ⁰C and has a pressure of 205.5 kPa. What mass of mercury metal is in the flask when the reaction is complete?

  A)  7.11 g*
  B)  4.33 g    
  C)  3.56 g    
  D) 17.1 g    
  E)  8.66 g

Solution

• Students needed to recognize the importance of calculating the number of moles of air in the reactant system before mercuric oxide decomposes.
• Using PV=nRT find number moles of air reactant: 0.024864 mol air at 21.0 ⁰C and 101.3 kPa.                                                                                               
• Then calculate the number of moles of gas in the product system. (0.042589 mol gas at 75.2 ⁰C and 205.5 kPa).   
• Recognize that the product system contains both the oxygen gas produced and the air that was originally in the system and subtract the number of moles of air from the number of moles of product gas, to determine the number of moles of oxygen gas produced.
• Moles of gas product - moles of air reactant. (0.017725 mol oxygen)
• Calculate the number of moles of mercury using the stoichiometry of the equation. (0.03545 mol Hg)
• Use the molar mass of mercury to calculate the mass of mercury. 

Most students failed to calculate the original number of moles of gas in the reactant mixture and subtract it from the number of moles of gas in the product mixture and instead used the answer from equation (2) for the number of moles of O₂. They incorrectly submitted answer letter D despite their correct application of the stoichiometry of the equation. 

What is the significance of the decomposition of mercuric oxide other than providing a tough CCC question? It is an opportunity to discuss a great story in the history of chemistry. The pastor turned chemist, Joseph Priestley, who discovered pencil erasers while trying to preserve paper in the many drafts of his drawings for his book about electricity, produced the first carbonate beverages by infusing water with carbon dioxide, and produced oxygen when he focused sunlight through a lens on mercuric oxide. Priestley recorded the emission of the hot gas from the red HgO powder and described the gas as part of the troubling phlogiston theory. The phlogiston theory explained that when substances burned, they released an invisible material substance called phlogiston. Air carried away the phlogiston and the ash left after combustion was dephlogisticated. To make the flawed phlogiston theory shoe fit better, Priestley dubbed oxygen ‘dephlogisticated air’ (say that 5 times fast). Scientists did not yet understand that 21% of the air around us is oxygen and Priestley noticed that dephlogisticated air had properties that differed from ‘pure’ air. You can read about Priestley’s discovery, Lavoisier’s sly theft of Priestley’s ideas and the subsequent attempt to claim the discovery of oxygen as his own, in Sam Kean’s latest book Caesar’s Last Breath: Decoding the Secrets of the Air Around Us.³  

Joseph Priestley, From Alfred E. Beach The Science Record for 1875 (New York: Munn and  Company, 1875)
 

Why do so many famous scientists seem to use some of the most toxic substances in many of the greatest discoveries? Mercury is a fascinating metal and before people understood the dangers; it is not really that surprising that experiments using or producing mercury were of interest to scientists. Mercury is not commonly found in its pure form in nature; however, archeologists found the metal in Egyptian tombs from as early as 1500 BC. Mercuric sulfide is the most commonly found mercury compound and was ground up and used as a red pigment in paints, dyes and cosmetics, from ancient Greek times to the mid-20th century. Mercuric oxide is less common. Despite financial help from a rich patron, one might wonder how Priestley, a poor preacher, happened upon the less common red powdery compound of mercury. Without HgO he may have never isolated oxygen and continued the revolution in the way humanity thinks about gases.

References

1a)    www.cheminst.ca/canadian-chemistry-contest-registration 
1b)    The CCC exams at www.cheminst.ca/outreach/canadian-chemistry-contest.
2)   K. Quaigrain and A. K. Arhin, Using reliability and item analysis to evaluate a teacher-developed test in educational measurement and evaluation. Cogent Education, 2017  

3)  S. Kean, Caesar's Last Breath, New York: Little, Brown and Company, 2017.

Acknowledgements

A team of eight chemistry educators from across Canada collaborated in the creation of Part A of the 2017 CCC. The 2017 CCC contributors were Dr. Sean Clapham, Dr. Kathy Darvesh, Dr. Andy Dicks, Ken Hoffman, Jennifer Howell, Suzanne Monir, Jenny Pitt-Lainsbury and Mengting Qiu. Sarah Cescon, Dr. John Sichel, Dr. Guillaume Bussiere and Dr. Denis Bussiere and the original contributors vetted and edited the questions. Dr. Denis Bussiere translated the exam into French and Dr. John Sichel edited the French exam. CIC gratefully acknowledges the volunteer contributions of the writing team in providing Canadian chemistry students the opportunity to develop their academic self-concept in chemistry and compete for monetary prizes. The CIC also thankfully acknowledges the time and effort of the District and Regional Coordinators in marking the exams and the contributions of the donors in providing the monetary prizes.