If the partial pressure of water vapour at 27°C is 18 mm Hg and the saturated vapour pressure of the atmosphere at the same temperature is 24 mm Hg, the rel...

Answer Details

Relative humidity is the ratio of the actual amount of water vapor present in the air to the maximum amount of water vapor the air can hold at a given temperature. It is usually expressed as a percentage. In this question, the partial pressure of water vapor in the air is 18 mm Hg, and the saturated vapor pressure of the atmosphere at the same temperature is 24 mm Hg. To find the relative humidity, we need to compare the partial pressure of water vapor in the air to the saturated vapor pressure of the atmosphere at the same temperature. The ratio of the two pressures is: Relative humidity = (partial pressure of water vapor / saturated vapor pressure) x 100% Substituting the values given in the question, we get: Relative humidity = (18 mm Hg / 24 mm Hg) x 100% = 0.75 x 100% = 75% Therefore, the relative humidity at this temperature is 75%. The correct option is C.