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Ch.12 - Liquids, Solids & Intermolecular Forces
Chapter 12, Problem 92

The vapor pressure of CCl3F at 300 K is 856 torr. If 3.5 g of CCl3F is enclosed in a 1.0-L container, will any liquid be present? If so, what mass of liquid?

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1
Calculate the number of moles of CCl_3F using its molar mass.
Use the ideal gas law, PV = nRT, to calculate the pressure exerted by the gas in the container.
Compare the calculated pressure with the given vapor pressure of CCl_3F at 300 K.
If the calculated pressure is greater than the vapor pressure, some liquid will be present. Calculate the moles of gas that would exert the vapor pressure.
Subtract the moles of gas from the total moles to find the moles of liquid, then convert to mass using the molar mass.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase at a given temperature. It indicates the tendency of a substance to evaporate; higher vapor pressure means a greater tendency to vaporize. In this question, the vapor pressure of CCl<sub>3</sub>F at 300 K is 856 torr, which serves as a critical reference point for determining if liquid will remain in the container.
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Ideal Gas Law

The Ideal Gas Law relates the pressure, volume, temperature, and number of moles of a gas through the equation PV = nRT. This law allows us to calculate the number of moles of CCl<sub>3</sub>F in the container, which is essential for comparing the amount of gas present to the vapor pressure. Understanding this relationship helps determine whether the conditions favor the presence of liquid or if the substance will remain entirely in the gaseous state.
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Phase Equilibrium

Phase equilibrium occurs when the rates of evaporation and condensation of a substance are equal, resulting in a stable mixture of liquid and vapor. In this scenario, if the number of moles of CCl<sub>3</sub>F exceeds what can exist as vapor at the given vapor pressure, some of it will condense into liquid. Analyzing the conditions of the system helps predict whether liquid will be present and, if so, how much.
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