| Q: | Which of the following compounds would be expected to have the highest boiling point?
|
| A | | 0.6m dextrose |
| B | | 0.3m sodium chloride |
| C | | 0.2m calcium chloride |
| D | | 0.2m sodium phosphate |
The correct answer is D. To solve, we must know how to calculate the Van’t Hoff factor. To do that, we need to determine first whether or not a substance ionizes in solution. Since dextrose does not, it has a Van’t Hoff of 1. The product of the Van’t Hoff factor and the molarity is the boiling point elevation. So 0.6m dextrose has a boiling point elevation of 0.6. Sodium chloride ionizes to one sodium molecule and one chloride molecule for a Van’t Hoff factor of 2. So 0.3m sodium chloride has a boiling point elevation of 0.6. Calcium chloride (CaCl2) ionizes to 1 molecule of calcium and 2 of clorine. So 0.2m calcium chloride has a Van’t Hoff factor of 3, and a boiling point elevation of 0.6. Sodium phosphate (Na3PO4) ionizes to 3 molecules of sodium and one of phosphate. So 0.2m of sodium phosphate has a Van’t Hoff factor of 4 and a boiling point elevation of 0.8, which is the highest of the four options here.