G chem question, cannot visualize

[sfoksn]

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A mixture of nitrogen and oxygen gas is collected by displacement of water at 25 degrees C and 600 torr Hg pressure. If the partial pressure of nitrogen is 500 torr Hg, what is the partial pressure of oxygen? (Vapor pressure of water at 25 degrees C = 30 torr)

a. 30
b. 120
c. 490
d. 70
e. 110

What situation is this? and how to think about it?

 

[qkchen]

is the answer d. 70 torr Hg?

my reasoning is this: total pressure is 600 torr Hg. Nitrogen contributes partial pressure of 500 torr Hg while water has vapor pressure of 30 torr Hg. the difference 600 torr Hg - 500 torr Hg - 30 torr Hg is the partial pressure of oxygen.

 

[sfoksn]

Yes, I got the same answer and that is the correct one, but I just wanted to be able to visualize the situation..

What exactly is going on in the situation?

 

[UCB05]

Yes, I got the same answer and that is the correct one, but I just wanted to be able to visualize the situation..

What exactly is going on in the situation?

total pressure is the sum of the partial pressures of all gases present. The question states that Nitrogen and Oxygen are collected over water. Whenever you hear that, it should click that you also factor in the partial pressure of water vapor. Therefore total pressure = p(nitrogen) + p(oxygen) + p(water). total pressure, p(nitrogen), and p(water) are all given, so it's just a matter of plug-and-chug to find p(oxygen)

edit* if you're having trouble visualizing what's going on, imagine an upside down cup in a tank of water. Nitrogen and oxygen bubble up into the cup, so there's a pocket of air made up of nitrogen, oxygen, and water vapor.