Finding water vapor content from pressure change at constant temperature

[apchemstudent]

I don't seem to be able to get close to any of the choices given.

Here's my work: 288 K = T
P = 101.23 kPa

2% volume = water vapor

Well the next day the pressure changes to 100.47 kPa at the same temperature. The volume changes by 101.23/100.47 = 1.008...

I'm stuck here... Can some one help? Thanks

 

[apchemstudent]

any idea on how to solve this problem?

 

[Hessam]

...

Well the next day the pressure changes to 100.47 kPa at the same temperature. The volume changes by 101.23/100.47 = 1.008...

...

pressure and volume are not directly proportional... perhaps this helps?

 

[GCT]

by avogadro's law the volume percent is equivalent to the mole percentage. In addition the mole percentage is equal to the pressure percentage (by dalton's law of partial pressures). Keep this in mind for the rest of this post.

I got one percent.
$$[100.47-(101.23-.02(101.23))]/100.47$$

I'm still not quite sure why they gave the molecular weight of gases in the problem, PV=nRT does not depend on mass composition. It's a colligative property

Once you find the final answer, please be considerate enough to tell the rest of us.