# 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?

Related Biology and Chemistry Homework Help News on Phys.org
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.