Question

What is the boiling point (in K) of the urea solution from the given graph?

• A. \( T_1 \)
• B. \( T_2 \)
• C. \( T_3 \)
• D. \( T_4 \)

Hint:

A non-volatile solute (like urea) raises the boiling point of a solution compared to the pure solvent.

Answer 1

The Correct Option is C

Step 1: Understanding Boiling Point Elevation 
- Boiling point is when the vapour pressure = 1 atm. 
- Water boils at \( T_2 \). 
- Urea solution has a higher boiling point due to colligative properties. 
- From the graph, \( T_3 \) corresponds to the urea solution’s boiling point.

Answer 2

Step 1: Understanding Boiling Point Elevation
- The boiling point of a liquid is the temperature at which its vapour pressure equals the external atmospheric pressure, typically 1 atm at sea level.
- Pure water boils at temperature \( T_2 \) under these standard conditions.

Step 2: Effect of Solute on Vapour Pressure
- Adding a non-volatile solute like urea to water lowers the vapour pressure of the solution because solute particles disrupt the escape of solvent molecules into the vapour phase.
- This lowering of vapour pressure means the solution will not boil at the same temperature as pure water.

Step 3: Boiling Point Elevation Explained
- Since the vapour pressure is lowered, the solution must be heated to a higher temperature to reach 1 atm vapour pressure.
- This increase in boiling point relative to the pure solvent is called boiling point elevation, a colligative property dependent on solute concentration, not its identity.

Step 4: Interpreting the Graph
- On the graph, \( T_3 \) represents the boiling point of the urea solution, which is higher than \( T_2 \), the boiling point of pure water.
- This visually demonstrates how the presence of urea raises the boiling point.

Step 5: Quantitative Aspect
- The boiling point elevation (\( \Delta T_b = T_3 - T_2 \)) can be calculated using the formula: \( \Delta T_b = K_b \times m \), where \( K_b \) is the ebullioscopic constant of the solvent and \( m \) is the molality of the solution.
- This relationship helps determine molar masses or concentrations in experimental settings.

Step 6: Practical Applications
- Boiling point elevation explains why salt is added to water while cooking—it raises the boiling point, allowing food to cook at higher temperatures.
- It is also crucial in industrial processes and formulation of antifreeze solutions.