I have a test on gas laws tomorrow, but I am unsure about onething. With Dalton\

Question

I have a test on gas laws tomorrow, but I am unsure about onething.
With Dalton's law, when finding the volume of the DRY gas of a gasthat is collected over water, do I subtract the vapor pressure fromBOTH of the given pressures or just the first?
Here's an example:
"If 500 mL of oxygen gas are collected over water at 20°C and745 torr, what will the volume of the dry gas be at STP? "
In this problem, would I subtract the vapor pressure (17.5 torr)from both the 745 torr AND the standard pressure of 760 torr, orwould I just leave the standard pressure alone and only subtractfrom the 745 torr before I work the problem?

Explanation / Answer

When oxygen is collected over water, the actual pressure ofoxygen = Measured pressure (745 torr) - vapor pressure of water atthat temperature (17.5 torr) From volume of gas(500 ml), pressure and temperature and R,calculate value of n - number of moles, Now at STP, use this value of n to calculate volume with STPvalues. So standard pressure has nothing to do with the subtraction atall. Hope you got it.

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