Re: Why are these tough questions?

In the September 2014 issue, we asked readers for their feedback on five items from last year’s CHEM 13 NEWS Exam. Based on item analysis these questions were considered good discriminators between strong and weak students. Here are some of your responses. To refresh your memory from September the items are listed to the right.

  • I’m sure many will respond with this thought, but at first glance I noticed that the questions require solutions with multiple steps and/or multiple concepts. From that observation I would hypothesize that weak students do poorly because they have multiple chances to make mistakes.

Curtis A. Musser
Viewpoint School, Calabasas CA

  • I read with interest the article in September’s issue of Chem 13 News. The five questions cited as discriminators between stronger and weaker students are all excellent questions in their own right and certainly well within the syllabus of Ontario Chemistry. I can see a number of reasons why they could stump weaker students.

Questions 8 and 14 involve gas laws and simple stoichiometry. They assess background material as opposed to current concepts. While most students will recall PV = nRT, few may have derived PM = DRT from it. A stronger student with good skills could deduce the result from the data given, but a weaker one will not know how to adjust the information to the known formula. Question 14, a simple limiting reagent question, should have been considered easy. No student having completed their first chemistry course should have had difficulty save, perhaps, not remembering the mole ratios in the balanced equation.

Questions 18 and 35 are “textbook” questions and are examples that would likely be attempted in most classrooms. The challenge in 18 may be the lack of equilibrium equation and not knowing (but not needing) the formula for pyridine. Students not totally certain about handling this question could have been derailed by the missing information, but a strong student with adequate experience would have been undeterred. In question 35 perhaps it was the need to convert the grams per litre to molarity that would lead a weaker student to select the distractor as their answer. Stronger students have better numeracy and coping skills to work through such questions.

Of all the questions, number 20 would likely be the most challenging. In Hess’ Law problems, students are shown how to add equations sets and then determine the enthalpy change by summation. Knowing that the equilibrium constants are multiplied and not summed may not be something all students understand. Moreover, classroom problems generally consider one ionization step only so combining constants is not heavily practiced. Hence those not as well prepared will be distracted by the answer that involves the sum of the constants rather than the product.

However, there may be other factors besides the questions themselves that distinguish student ability. In my experience of offering both the CHEM 13 NEWS and the Avogadro Exams to students (extending now over 40 years), May is a busy time for students, and it is rare to find one that adequately prepares for the contest. Offered as an option and never run as a class during class time, practice was always a student initiative. Lack of adequate preparation would certainly affect performance by students with weaker recall ability for questions 8 and 14.

Semestered schools are always at a disadvantage as, by early May, there is still much to cover and serious practice must be undertaken to cover the missing units. This is not a criticism, but a fact of life. If schools want results in a timely manner, then the exam must be written in early May. An early May writing for a second semester student would put the last three questions in jeopardy. Semester 1 students will have covered the course, but without advanced preparation weaker students will have limited recall.

Finally, students take Ontario grade 12 Chemistry (or equivalent) for varying reasons. Not all of these are because they enjoy or excel at the subject. If schools were to screen candidates and only allow the best and brightest to write, one should expect superior scores and fewer missed questions of the types listed. However, if schools leave contest writing open to anyone who chooses to, students who are less capable, have weaker levels of knowledge and skills and who take less time to prepare will also write. Thus there will be those for whom the distractors will appear correct and these questions will continue to discriminate.

In a free society the contest should be open to everyone. But the downside is that some excellent and very fair questions will be answered poorly.

Lyle Sadavoy (retired)
Toronto ON 

[Teachers are encouraged to try out these questions (below) with their students. As always, the entire 2014 exam plus answers are posted on the CHEM 13 NEWS Exam website.

CHEM 13 NEWS Exam will be written on Thursday, May 14, 2015. Avogadro Exam will be written on Thursday, May 21, 2015.

Information will be sent out in mid-February 2015 and the ordering deadline will be in early April 2015.

(Chem 13 News, September 2014)

8)  The density of an ideal gas at 90oC and 94.0 kPa is
2.80 g/L.

What is the molar mass of the gas?

  1. 23 g/mol
  2. 45 g/mol
  3. 90 g/mol
  4. 180 g/mol
  5. 270 g/mol

14)  What is the maximum mass of KBr that can be obtained
from a mixture containing 13.2 grams of potassium (K) and
22.2 grams of bromine (Br2)?

  1. 13.2 g
  2. 22.2 g
  3. 33.1 g
  4. 35.4 g
  5. 40.2 g

18)  A solution of 0.060 mol L−1 pyridine (a weak base) has
[OH ] = 9.1×10−6 mol L−1. What is Kb for pyridine?

  1. 1.5 × 10−4
  2. 3.0 × 10−4
  3. 8.3 × 10−11
  4. 9.1 × 10−6
  5. 1.4 × 10−9

20)  The equilibrium constants for the following reactions are
denoted by K1K2, and K3, respectively.

HNO2(aq) + H2O(l) ⇄ NO2-(aq) + H3O+(aq)         K1

2 H2O(l) ⇄ H3O+(aq) + OH-(aq)                                K2

NH3(aq) + H2O(I) ⇄ NH4+(aq) + OH-(aq)              K3

What is the equilibrium constant for the reaction below?

HNO2(aq) + NH3(aq) ⇄ NO2-(aq) + NH4+(aq)

  1. K1 – K2 + K3
  2. K1 K3
  3. K1 K3 / K2
  4. K1 K2 K3
  5. K2 / (K1 K3)

35)  The solubility of MnS in water is 2.3×10−6 grams per litre.
What is Ksp for MnS?

  1. 4.9 × 10−31
  2. 2.8 × 10−23
  3. 5.3 × 10−12
  4. 7.0 × 10−16
  5. 2.3 × 10−6