# Vapor pressure

#### Temperature101

##### Membership Revoked
Removed
Hydrogen gas generated in a reaction is collected over water at 25 degrees Celsius and is found to have a pressure of 746 torr. The vapor pressure of water at 25 degrees Celsius is 24 torr. Which expression can be used to find the pressure of H2, in atm?
B. 746/760

This question is from mcatquestionoftheday.com... can someone explain it? The answer is D. It seems to be an easy question. But I dont understand why they have to subtract 24 from 746 when they already gave the pressure of H2.​

#### piojita63

##### Full Member
Hydrogen gas generated in a reaction is collected over water at 25 degrees Celsius and is found to have a pressure of 746 torr. The vapor pressure of water at 25 degrees Celsius is 24 torr. Which expression can be used to find the pressure of H2, in atm?
B. 746/760

This question is from mcatquestionoftheday.com... can someone explain it? The answer is D. It seems to be an easy question. But I dont understand why they have to subtract 24 from 746 when they already gave the pressure of H2.​
I guess that when they say that the gas collected has a pressure of 746 torr, they mean that it is NOT PURE H pressure. (It is H and H20 vapor pressure together). You can infer that from the question since they say that water DOES have vapor pressure at that same temperature.

#### pfaction

##### Full Member
10+ Year Member
Water is exerting a vapor pressure as the H2 is collected. Therefore to find pure H2, substract the contaminant.

If you went to pick up blue sticks and also picked up a red stick, you'd substract the red sticks from the total amount of sticks to know the number of blue sticks.

#### Temperature101

##### Membership Revoked
Removed
Water is exerting a vapor pressure as the H2 is collected. Therefore to find pure H2, substract the contaminant.

If you went to pick up blue sticks and also picked up a red stick, you'd substract the red sticks from the total amount of sticks to know the number of blue sticks.
Lol.....

##### MD
Moderator Emeritus
10+ Year Member
Yeah, this is just testing if you know what "collected over water" means. What it literally means is that if you're collecting the gas in a balloon that is secured over a beaker/test tube/whatever, and the gas is evolving from the aqueous solution in the container, as it bubbles out there is necessarily some amount of gaseous water that has escaped from the liquid solution at the given temperature.

#### milski

##### 1K member
5+ Year Member
Except they told me that I've collected 746 blue sticks, not 746 sticks. A bit of a question - they could work on the phrasing.

##### MD
Moderator Emeritus
10+ Year Member
Except they told me that I've collected 746 blue sticks, not 746 sticks. A bit of a question - they could work on the phrasing.
I get what you mean, but IIRC all the questions in my gen chem book that said "collected over water" even if they "collected X gas" used the phrase to mean that some of the measured pressure was due to H2O(g).

This thread is more than 9 years old.

Your message may be considered spam for the following reasons: