# Question on water vapour and pressures

## Homework Statement

20 litres of moist air at 25 degrees celsius with a relative humidity of 75% are compressed isothermally to a volume of 4 litres. Calculate the mass of water condensed? (One method, find the original water vapour pressure, it will be increased by what factor?)

## Homework Equations

pV= mRT

water vapour pressure = density x R x T

## The Attempt at a Solution

Where should I start with this problem. Finding the mass is really hard here, the mass of water condensed is only part a of the question. if i had that, i could figure it all out. any hints in the right direction?

Borek
Mentor
What is pressure of saturated water vapour at 25 deg C?

sorry borek u lost me. am i supposed to read the pressure of saturated water vapour at 25 degrees off a chart? Because right now, I cannot see an equation that help me find the mass with the current information provided.

yea think i got it. I used the chart to search for saturated vapour pressures at 25 deg C, then used PV= mRT to calculate the mass of water. Then use relative humidity to find mass of water condensed.

cheers anyway

Borek
Mentor
am i supposed to read the pressure of saturated water vapour at 25 degrees off a chart?

Yes.