The vapour pressure vs. temperature curve for a solution solvent system is shown below.
The boiling point of the solvent is __________ ° C.

Solution

Chemistry ma’am · 3 steps

The solvent is the upper curve: pure solvent always has the higher vapour pressure, so it reaches 1 atm first.

1. Which curve is which. A non-volatile solute lowers the vapour pressure (Raoult's law). So at every temperature the pure solvent sits above the solution. The upper curve is the solvent and the lower curve is the solution.
2. Boiling point. A liquid boils when its vapour pressure equals the outside pressure, 1 atm. Read off where each curve meets the dashed 1 atm line.

   Curve	Vapour pressure	Meets 1 atm at
   Upper: solvent	higher	82 °C
   Lower: solution	lower	83 °C

3. So. The solvent boils at 82 °C. The solution boils 1 °C higher, which is the elevation in boiling point.

The common slip: 83. Reading the lower curve gives 83, but that is the solution's boiling point. The solute lowers the vapour pressure, so the solution's curve is the lower one.

The idea to keep. On a vapour pressure graph, the pure solvent's curve lies above the solution's. Boiling point is where a curve meets the external pressure line, and the solution always reaches it later.

Answer from the NTA final answer key. Solution by PrepWiser.