You are Here: Home

# Seemingly simple gas/pressure question Tweet

Physics and electronics discussion, revision, exam and homework help.

Announcements Posted on
TSR launches Learn Together! - Our new subscription to help improve your learning 16-05-2013
IMPORTANT: You must wait until midnight (morning exams)/4.30AM (afternoon exams) to discuss Edexcel exams and until 1pm/6pm the following day for STEP and IB exams. Please read before posting, including for rules for practical and oral exams. 28-04-2013
1. Seemingly simple gas/pressure question
A hand pump of volume 2.0 x 10-4 m3 is used to force air through a valve into a container of volume 8.0 x 10-4 m3 which contains air at an initial pressure of 101kPa. Calculate the pressure of the air in the container after one stroke of the pump, assuming the temperature if unchanged.

Firstly I assume that this question involves Boyle's law, which is PV = k (where P = pressure, V = volume, k = constant). I found the constant by subbing in the initial pressure and the container's volume to get 80.8. I then subbed in this constant value, and the volume of 8.0 x 10-4 m3 minus 2.0 x 10-4 m3, because I figured that if air is being pumped in, then the container's volume must be reduced as the pumped in air takes up space. However I get pressure as 134kPa, when the answer at the back have it as 126kPa.

Honestly I'm stuck, and this is supposed to be an easy starter question as well
2. Re: Seemingly simple gas/pressure question
Boyle's law is for a constant quantity of gas. You need the full ideal gas equation, which is

PV = NkT

where k is the Boltzmann constant. What you're changing is the number of molecules in the container, not it's volume.

Presumably the hand pump also contains air at an initial pressure of 101kPa, so you can just look at what fraction of the container's original volume is being added to it. That should give you the answer you're looking for.
3. Re: Seemingly simple gas/pressure question
(Original post by BJack)
Boyle's law is for a constant quantity of gas. You need the full ideal gas equation, which is

PV = NkT

where k is the Boltzmann constant. What you're changing is the number of molecules in the container, not it's volume.

Presumably the hand pump also contains air at an initial pressure of 101kPa, so you can just look at what fraction of the container's original volume is being added to it. That should give you the answer you're looking for.
Whats 'N' and 'T'? in the equation? T means temperature, right? - how does this come in, because temperature is unchanged throughout
4. Re: Seemingly simple gas/pressure question
(Original post by internet tough guy)
Whats 'N' and 'T'? in the equation? T means temperature, right? - how does this come in, because temperature is unchanged throughout
N is the number of molecules you have. T is the temperature; and, yes, it's being held constant so you can ignore that factor.
5. Re: Seemingly simple gas/pressure question
Surely you'd use P1V1 = P2V2?

(8.0 x 10-4 + 2.0 x 10-4)(101kPa) = (8.0 x 10-4)(P2)

Rearrange et voila; you have your answer of 126kPa