Question

The ratio of mole fraction of water vapor in a given moist air sample to the mole fraction in an air sample saturated at the same temperature and pressure is known as

• A. Relative Humidity
• B. Specific Volume
• C. Wet Bulb Temperature
• D. Dew-Point Temperature

Hint:

Remember the two main humidity types: \textbf{Relative Humidity} is a {ratio} (how full the air is with water, from 0% to 100%). \textbf{Specific Humidity} (or humidity ratio) is an {absolute amount} (mass of water per mass of dry air).

Answer 1

The Correct Option is A

Step 1: Define Relative Humidity (RH). Relative humidity is a measure of the amount of moisture in the air compared to the maximum amount of moisture the air can hold at that temperature.

Step 2: Express the definition mathematically. For ideal gases, the partial pressure of a component is proportional to its mole fraction ($p_i = y_i P_{total}$). Therefore, the ratio of partial pressures is equal to the ratio of mole fractions.
\[ RH = \frac{\text{Partial pressure of water vapor}}{\text{Saturation pressure of water vapor at same T}} = \frac{p_w}{p_{ws}} \] Since $p_w = y_w P_{total}$ and $p_{ws} = y_{ws} P_{total}$: \[ RH = \frac{y_w P_{total}}{y_{ws} P_{total}} = \frac{\text{Mole fraction of water vapor in sample}}{\text{Mole fraction of water vapor in saturated sample}} \] This exactly matches the definition given in the question. The other terms are different concepts: Specific Volume is volume per unit mass, and Wet Bulb and Dew-Point are temperatures.