Partial pressure question

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!