Final answer:
Without additional data (specifically the temperature and the vapor pressure of each component), it's not possible to accurately calculate the pressure at which 30% of the vapor will liquefy in an ideal solution of A and B, as it requires application of Raoult's Law and possibly the Clausius-Clapeyron equation.
Step-by-step explanation:
The student's question pertains to the physical chemistry concept of vapor-liquid equilibrium in an ideal solution. The scenario given involves a cylinder and piston arrangement with a mixture of vapors of two substances, A and B. Initially, the vapor pressures of A and B combined equal 300 torr. The student is asking for the pressure at which 30% of the total amount of vapor present will liquefy.
To find the pressure at which a certain fraction of an ideal gas mixture liquefies, we must take into account Raoult's Law. Raoult's Law states that the partial pressure of each component in an ideal solution is directly proportional to its mole fraction. However, it is necessary to have information about the conditions at which the liquids form: the temperature and the vapor pressure of each component at that temperature.
Giving a specific pressure without the aforementioned data would be speculative. Instead, we can discuss the general approach. If 30% of the vapor is to become liquid, we should withdraw heat from the system at constant temperature until the reduced vapor pressure corresponds to this new state. The Clausius-Clapeyron equation may be used when dealing with phase changes to relate the pressure of the system to the temperature and the heat of vaporization. Nevertheless, the specific data required to perform these calculations are not provided in the question.