Question

Assuming that a different air mass has a relative humidity of 40% at a temperature of 25 degrees Celsius, what is the actual humidity content, expressed as g/kg?

Answer 1

The relative humidity is defined by the following expression (http://www.classzone.com/vpg_ebooks/ml_earthscience_na/accessibility/ml_earthscience_na/page_392.pdf):

$$RH = \dfrac{Actual\space humidity}{Maximum \space humidity}\times 100\%$$

where $Maximum \space humidity$ is the maximum gramms of water that one kilogram of air can hold at given temperature (units $g/kg$ ). For temperature $25\degree C$, have (see https://www.engineeringtoolbox.com/humidity-ratio-air-d_686.html):

$$Maximum \space humidity = 19.8\space g/kg$$

Thus, the $Actual\space humidity$ is:

$$Actual\space humidity = \dfrac{RH\times Maximum \space humidity}{100\%} = \dfrac{40\%\times 19.8}{100\%}=\\ =7.92\space g/kg$$

Answer. $7.92\space g/kg$.