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Relative humidity is a measure that tells us how much water vapor is present in the air compared to the maximum amount of water vapor that the air can hold at a specific temperature. It is expressed as a percentage. To understand this better, imagine that warm air can hold more moisture than cold air. Thus, if the air is warm, the air might only be filled with a certain percentage of water vapor, representing a lower relative humidity.
Relative humidity can be calculated using the formula:

• \text{Relative Humidity} = \frac{P_w}{P_s} \times 100 \%

Where:

• \(P_w\) is the partial pressure of water vapor.
• \(P_s\) is the saturation vapor pressure at that particular temperature.

This percentage shows the ratio of current vapor pressure to the vapor pressure at which the air would be saturated. When the relative humidity is 100%, the air is completely saturated, indicating that any additional moisture would result in condensation.

The ideal gas law is a fundamental equation in chemistry that relates the pressure, volume, temperature, and number of moles of a gas. It is given by the formula:

• \( PV = nRT \)

Where:

• \(P\) is the pressure of the gas.
• \(V\) is the volume the gas occupies.
• \(n\) is the number of moles of the gas.
• \(R\) is the ideal gas constant (8.314 \(J/mol \cdot K\)).
• \(T\) is the temperature in Kelvin.

This law helps us understand and predict the behavior of a "perfect" gas under various conditions.
In real-world calculations, like determining the mass of water vapor in air, the ideal gas law is modified slightly to consider the vapor pressure as the pressure in the equation, allowing us to solve for the number of moles and, subsequently, the mass of the vapor.

Vapor-liquid equilibrium is a condition where a liquid and its vapor exist together in a closed system without any net change over time. Imagine a closed container with water; as water molecules evaporate into vapor, some vapor molecules condense back into liquid. Over time, a balance is reached where these processes occur at the same rate, creating an equilibrium.
This concept is crucial in understanding vapor pressure, the pressure exerted by the vapor when it is in equilibrium with its liquid at a given temperature. At this point, both phases are stable because the rates of evaporation and condensation are equal.
Vapor-liquid equilibrium explains why vapor pressure increases with temperature: more molecules have enough energy to escape the liquid phase. In calculations, it helps us determine conditions like the relative humidity, indicating how close the system is to saturation.