Question

A glass contains some water at room temperature 20°C. Refrigerated water is added to it slowly. when the temperature of the glass reaches 10°C, small droplets condense on the outer surface. Calculate the relative humidity in the room. The boiling point of water at a pressure of 17.5 mm of mercury is 20°C and at 8.9 mm of mercury it is 10°C.

Answer 1

The relative humidity in the room is 51%

Explanation:

Step 1:

given data in the question  

Dew appears  = $10^{\circ} \mathrm{C} \quad\left(\text { Dew point at } 10^{\circ} \mathrm{C}\right)$

Saturated vapor pressure equals atmospheric pressure at boiling point.

to $20^{\circ} \mathrm{C}$ ,  

Saturated vapour pressure ( SVP ) = $17.5 \mathrm{mm} \mathrm{Hg}$

Step 2:

At  dew point, Saturated vapour pressure ( SVP )  = $8.9 \mathrm{mm} \mathrm{Hg}$

$R H(\text { Relative humidity })=\frac{\text { Saturated vapour pressure } a t \text { dew point }}{\text { Saturated vapour pressure at air temperature }} \times 100 \%$

$=\frac{8.9}{17.5} \times 100$

$=0.508 \times 100$

$=50.8 \approx 51 \%$