# Why are these tough questions?

Each year after the dust has settled, Carey Bissonnette at the University of Waterloo reviews the CHEM 13 NEWS Exam statistics. As part of an item analysis, a point-biserial correlation coefficient is calculated for each of the 40 exam items (questions). Basically, this is the correlation that quantifies the relationship between how students do on each individual item versus the overall exam. An item that has a point-biserial closer to 1.0 indicates that students who did well on the exam also did well on that particular item, while student who did poorly on the item were likely to do poorly on the entire exam. If, and this would not be good, an item has a negative correlation, it indicates that good students did poorly on that item and weak students did well. This could be explained by the item being “tricky” and somehow confused the better students. I am sure most teachers have come across a question or two that would fall into this category in their own teaching.

Below are items from the exam that had a point-biserial greater than 0.5. These items can be considered good discriminators between “stronger” students and “weaker” students.

Read through these items and see if you agree with the statistics. We would love to hear feedback from high school teachers on why you think these items discriminate. What in particular makes the concepts covered in these items so fragile that weaker students find them difficult? What are you doing in your own teaching to address these difficulties? Send your comments to Jean Hein, jhein@uwaterloo.ca and be part of the discussion.

As always, the entire 2014 exam plus answers are posted on the CHEM 13 NEWS Exam website.

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 K1, K2, 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