[anthony_ball]

i want to ask some questions about PV = NRT

sometimes , we put V = volume of gas  with P = external pressure
for us to calculate the others quantity...

but i don't know why sometimes ..
we need to put V = volume of the container
and we can get the pressure exerted by the gas . ( the partial pressure )

thx all!

[typhoon2028]

sometimes , we put V = volume of gas  with P = external pressure
for us to calculate the others quantity...

Think about a balloon.  A balloon is not a rigid container.  A gas will fill a balloon to a certain volume, depending on the external pressure.  Assuming you ignore the force required to expand a balloon (from the elasticity).  The gas pressure inside the balloon will be equal to the external pressure.  For these types of problems.  Pressure will be held constant, and the variables become volume, temperature, and # moles.

but i don't know why sometimes ..
we need to put V = volume of the container
and we can get the pressure exerted by the gas . ( the partial pressure )

Here you have to think in terms of a rigid container, like a gas cylinder.  Volume will be fixed, and the problem variables become pressure, # moles, and temperature.

Partial pressure for an ideal gas mixture is proportional to mole fraction.

FYI, you should write the equation as:  PV = nRT.

[anthony_ball]

can you explain it further
i am still not clear about it

thx !!!

[Arkcon]

can you explain it further
i am still not clear about it

thx !!!

Well, lets try to figure it out stepwise, so it all becomes clearer for you to understand, and for us to see what you're missing.  What does each letter in the formula stand for?

[anthony_ball]

A gas will fill a balloon to a certain volume, depending on the external pressure ...

The gas pressure inside the balloon will be equal to the external pressure.

i can't understand about this .

why pressure is constant ??

Second ,
in the case of the rigid container...
it is east to understand that the volume is constant
but i don't know why the volume of gas = volume of container .
and the pressure calculated is related to the gas (not like in case 1 , the pressure is related to external)

sorry for my stupidity..

[Arkcon]

Well, it really is the definition of the ideal gas law, isn't it?  We visualize a gas, as a collection of discreet, mass-less particles, all with an average velocity.  Right. (Hint: what gives molecules, a velocity?  What do we call, molecules or atoms with a velocity?) They fly around, bouncing off each other, and the walls of their container, and we call that -- pressure.  You question, which you've simply repeated is hard to follow.  Does your text book have images, cartoons if you will, that are similar to what I've described.  And can you look up the ideal gas law, identify what the letters stand for, and try to pull together the various concepts?

[typhoon2028]

A gas will fill a balloon to a certain volume, depending on the external pressure ...

The gas pressure inside the balloon will be equal to the external pressure.

i can't understand about this .

why pressure is constant ??

First we must assume about the balloon, the rubber or whatever elastic material the balloon is made of does not resist the gas expansion.
I guess it would be incorrect to say pressure is constant.  It is better to say, in a balloon problem the pressure, will always be equal to the outside pressure.
Balloons change shape easily, and if the pressure did not equalize the shape would keep changing until the inside pressure equaled the outside pressure.

Second ,
in the case of the rigid container...
it is east to understand that the volume is constant
but i don't know why the volume of gas = volume of container .
and the pressure calculated is related to the gas (not like in case 1 , the pressure is related to external)

You have to understand or accept that gases and liquids fill and take the shape of their containers.