# gen chem, vapor pressure

#### dl9006

##### Full Member
10+ Year Member
5+ Year Member
Question1:

In a cylinder containing N2 which is insoluble in water, there is a small amount of water at 25 C. At this temp, water has vapor press of 24 mm Hg.
The total press of the cylinder at this temp is 600 mm Hg.

If the volume was reduced to one half, the partial pressure of the gas would be:
a) 2atm
b)1200 mm hg
c) 1176 mm hg
d) 48 mm hg
e) 24 mm hg

ans C
------------------------------------

It seems that the answer is derived by doubling the original pressure minus the 24 mm Hg from the water pressure. 600X2-24=1176mm Hg

My question is why doesn't the water vapor pressure double like the N2 pressure?

I understand that water is relatively incompressible but shouldn't the water vapor which is in gas form be compressed along with the N2 gas and have a pressure double its original?

Question 2:

vapor pressure of water at 100 C is measured on the moon. Compared to vapor pressure on earth at the same temp, the vapor pressure would be:

a)higher
b)lower
c)variable
d)dependant on the time of measurement
e) the same

ans: e) the same

I thought the vapor pressure would be higher on the moon since there's no atmosphere to prevent the water molecules from escaping its liquid phase.

Isn't this like how water boils at much lower temp at higher altitude?
Less atmospheric pressure, thus more water molecules escape and thus higher vapor pressure?

Last edited:

#### UCB05

##### Full Member
Question:

In a cylinder containing N2 which is insoluble in water, there is a small amount of water at 25 C. At this temp, water has vapor press of 24 mm Hg.
The total press of the cylinder at this temp is 600 mm Hg.

If the volume was reduced to one half, the partial pressure of the gas would be:
a) 2atm
b)1200 mm hg
c) 1176 mm hg
d) 48 mm hg
e) 24 mm hg

ans C
------------------------------------

It seems that the answer is derived by doubling the original pressure minus the 24 mm Hg from the water pressure. 600X2-24=1176mm Hg

My question is why doesn't the water vapor pressure double like the N2 pressure?

I understand that water is relatively incompressible but shouldn't the water vapor which is in gas form be compressed along with the N2 gas and have a pressure double its original?

If pressure increases, the water vapor can shift back into the liquid phase to keep the temperature-dependent vapor pressure of 24mmHg constant. the N2 has no place to go.

#### dl9006

##### Full Member
10+ Year Member
5+ Year Member
If pressure increases, the water vapor can shift back into the liquid phase to keep the temperature-dependent vapor pressure of 24mmHg constant. the N2 has no place to go.

oh right didn't think of that. thanks UCB!

i just edited the post and added a 2nd related question.

#### UCB05

##### Full Member
oh right didn't think of that. thanks UCB!

i just edited the post and added a 2nd related question.

Vapor pressure of a liquid is solely based on temperature. since the question explicitly asks for the vapor pressure at the same temperature on the earth and moon, vp has to be the same.

The phenomena of lowered bp at higher atmosphere is due to lowered atmospheric pressure and therefore partial pressure of water. Remember the bp is the point where the vp of a liquid is equal to the atmospheric vp. Water boils at a lower temperature because of its relation to the surroundings. The vp of 98-degree water is the same at sea level and in the mountains, but it's enough to boil in the mountains, but not at sea level

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