Answer: The mole fraction of heptane at 80°C is 0.506
Step-by-step explanation:
We are given:
Vapor pressure of heptane at 80°C = 428 mmHg
Vapor pressure of octane at 80°C = 175 mmHg
Total pressure at 80°C = (428 + 175) = 603 mmHg
To calculate the mole fraction of heptane at 80°C, we use the equation given by Raoult's law, which is:

where,
= partial pressure of heptane = 305 mmHg
= total pressure = 603 mmHg
= mole fraction of heptane = ?
Putting values in above equation, we get:

Hence, the mole fraction of heptane at 80°C is 0.506