Partial Pressure Calculation for Oxygen in a Closed Container

[lzh]

1) Question:
A sample of oxygen is collected in a 172 mL
container over water at 15C, and the barom-
eter reads 754 torr. What volume would the
dry gas occupy at 778 torr and 15C? Water's
partial pressure at 15C is 12:8 torr. Answer
in units of mL.
2)attempt to answer:
pv=PV
temp constant
(754+12.8)*172=(778+12.8)V
V=166.779969
I'm not exactly sure if this is the right way to do it since the question's wording is a bit confusing and there seems to be some useless information. Can anyone tell me if my answer is right here?
Thanks

 

[eli64]

you need to SUBTRACT the partial pressure of water from the first pressure of 754 torr. (wet gas to dry gas) This is now p1, v1 = initial volume. P2 is the new pressure 778. find V2

 

[lzh]

thanks! i understand now!