PV or VP?

[MotoPayton]

I have a basic question on my prelab. The professor wants us to make a graph of of a experiment where the volume of methane gas was measured at various pressures. The temperature is held constant. She gives us data for this experiment and wants us to make a graph for it as practice.

I can dot the graph but is this correct? I have studied isotherms in the past and I have never heard of being able to change pressure and measure volume from it? I thought the volume was the independent set and pressure was dependent? Im confused

Thanks

[Borek]

Take cylinder and piston. Apply different forces. Pressure inside is a simple function of the force. You can measure volume from the piston position. Where is the problem?

[MotoPayton]

Can you change the pressure on a system and then measure the resultant change in volume? My question is wether the order matters in PV graph

Do you either
1) Change volume
2) measure pressure

or the opposite way
1)change pressure
2) measure volume

Because isn't it impossible to change the pressure without touching the volume in an isothermic process?

[Borek]

Can you change the pressure on a system and then measure the resultant change in volume?

Yes, as I explained to you in my previous post.

Do you either
1) Change volume
2) measure pressure

I can't think of a way to change the volume not by changing the pressure. But it doesn't matter much. Isotherm is described by pV=const. Both plots are correct. You don't have to change the volume - you may know the volume and be interested in what the pressure is.

[MotoPayton]

I can't think of a way to change the volume not by changing the pressure.

Allright cool that answers my question.
Thanks

[jtabije]

Recall the gas laws:

Boyle's Law states:
P\alphaV (The pressure of a gas is proportional to its volume and vice versa.)
Avogadro's Law states:
V\alphan (The volume of a gas is proportional to its amount (in moles) and vice versa.)
Charle's Law states:
V\alphaT (The volume of a gas is proportional to its temperature and vice versa.)
Gay-Lussac's Law states:
P\alphaT (The pressure of a gas is proportional to its temperature and vice versa.)

From these gas laws, you can derive the combined gas law equation:

\frac{P1V1}{n1T1} = \frac{P2V2}{n2T2}

..and since the process you were observing was isothermic (and assuming that no gas was added or removed), the equation will simplify to:

P1V1 = P2V2 where any change in pressure will result in a change in volume and vice versa.

Hopefully, in addition to Borek's replies, this will add a little bit more insight. :)

[PhaseShifter]

Recall the gas laws:

Boyle's Law states:
P\alphaV (The pressure of a gas is proportional to its volume and vice versa.)

Actually, you have this one wrong...pressure and volume are inversely proportional, not directly proportional.

[jtabije]

Actually, you have this one wrong...pressure and volume are inversely proportional, not directly proportional.

Well, isn't that embarrassing on my part? Thanks for the correction!