Answer

🧪 Problem Statement

5 moles of liquid X and 10 moles of liquid Y make a solution with a vapour pressure of 70 torr.
The vapour pressures of pure X and Y are 63 torr and 78 torr, respectively.

Question: Which of the following is true regarding the described solution?

📘 Concept: Raoult’s Law

For an ideal solution, the total vapour pressure is given by:

🧮 Step 1: Mole Fractions

• Total moles = 5 + 10 = 15
• X_X = 5 / 15 = 1/3
• X_Y = 10 / 15 = 2/3

🔍 Step 2: Ideal Vapour Pressure

Pideal = (1/3) × 63 + (2/3) × 78 = 21 + 52 = 73 torr

📉 Step 3: Comparison

• Actual Vapour Pressure = 70 torr
• Ideal Vapour Pressure = 73 torr

Since actual < ideal, the solution shows negative deviation.

💡 Step 4: Interpretation

Negative deviation occurs when:

• Intermolecular forces between components are stronger than in pure forms.
• Fewer molecules escape into vapour phase.
• Solution may show volume contraction and release heat (exothermic).

❌ Misconceptions

• Positive deviation: Occurs when actual vapour pressure is greater than ideal.
• Ideal solution: Follows Raoult’s Law exactly.
• Volume increase: Happens in positive deviations, not here.

✅ Final Answer

🔚 Conclusion

The solution has a vapour pressure lower than predicted by Raoult’s Law, indicating stronger interactions between X and Y molecules. Hence, it shows a negative deviation.