MCAT General Chemistry Question 131: Answer and Explanation
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Question: 131
11. Which of the following will cause the greatest increase in the boiling point of water when it is dissolved in 1.00 kg H2O?
- A. 0.46 mol calcium sulfate
- B. 0.54 mol iron(III) nitrate
- C. 1.09 mol acetic acid
- D. 1.11 mol sucrose
Correct Answer: B
Explanation:
The equation to determine the change in boiling point of a solution is as follows: ΔTb = iKbm.m is the molality of the solution, and Kb is the boiling point elevation constant. In this case, the solvent is always water, so Kb will be the same for each solution. What we do need to know is how many particles dissociate from each of the original species. This is referred to as the van 't Hoff factor (i) and is multiplied by our molality to demonstrate a normality (the concentration of the species of interest—in this case, all particles). We'll use normality values to determine which will cause the greatest change in boiling point.
| Species | Number of Moles | Number of Dissolved Particles | i × m (Normality) |
| CaSO4 | 0.46 | 2 | 0.92 |
| Fe(NO3)3 | 0.54 | 4 | 2.16 |
| CH3COOH | 1.09 | Between 1 and 2 (acetic acid is a weak acid and a low percentage of the molecules will dissociate into 2 particles) | Between 1.09 and 2.18 |
| C12H22O11 | 1.11 | 1 | 1.11 |
It looks like we need to decide between iron(III) nitrate and acetic acid. No more calculations are necessary because the fact that acetic acid is a weak acid tells us that only a few particles will dissociate into H+ and acetate. Therefore, the normality of the acetic acid will be much closer to 1.09 than 2.18.