Question

For a typical sample of ambient air $(at{30}^{\circ }C,75\%$ relative humidity and standard atmospheric pressure), the amount of moisture in kg per kg of dry air will be approximately.

• A. 0.002
• B. 0.027
• C. 0.25
• D. 0.75

Answer 1

The correct option is B 0.027

Since, in the problem, value of saturated vapour is not given. So taken the value of saturated vapour pressure at atmospheric condition is about

5 kPa. $i.e{p}_{vs}=5\text{kPa}$

Relative humidity,
$\varphi =75\%=0.75=\frac{p_v}{p_{vs}}$

Partial pressure of water in mixture,

$p_v=5\times 0.75=3.75\text{kPa}$

Amount of mixture in kg per kg of dry air i.e. specific humidity,

$\omega =\frac{0.622p_v}{p-p_v}=\frac{0.622\times 3.75}{\mathrm{1.01.325}-3.75}$

$=0.0239$kg/kg of dry air.