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Question 58

The vapour pressure vs. temperature curve for a solution solvent system is shown below.

image


The boiling point of the solvent is ______°C.


Correct Answer: 82

Solution

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The boiling point of any liquid is the exact temperature at which its vapor pressure becomes equal to the atmospheric pressure (given here as 1 atm on the y-axis).

Furthermore, because adding a non-volatile solute always decreases a liquid's vapor pressure, the curve located further to the left (higher vapor pressure at any given temperature) always represents the pure solvent.

Step 1: Identify the Pure Solvent Curve

Looking closely at the two parallel liquid-gas equilibrium lines:

  • The **Left-most Curve** represents the pure solvent.
  • The **Right-most Curve** represents the solution (due to vapor pressure lowering).
Step 2: Trace across from 1 atm

Follow the horizontal dashed reference line originating from 1 atm on the vertical axis:

  • It intersects the pure solvent curve (left curve) first. Dropping straight down to the temperature axis from this intersection point reads exactly 82°C.
  • It intersects the solution curve (right curve) second. Dropping down to the temperature axis from this intersection point reads exactly 83°C..
Correct Boiling Point of the Solvent: 82°C

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