[Norngpinky]

I'm pretty new on this forum, and thus not sure if this should be in the High School Chemistry section or the General one...This one is pretty general, however, even though it's AP...

So um...

I have a pressure problem in chem and I am not sure how to start this out.


"A 3.0 L bulb containing He at 145 mmHg is connected by a valve to a 2.0-L bulb containing Ar at 355 mmHg. (see the figure) Calculate the partial pressure of each gas and the total pressure after the valve between the flasks is opened."


So I put....

He ~ V=3.0L ; P=145 mmHg

Ar ~ V=2.0L; P=355 mmHg

After mixing those two bulbs
Total pressure = 145 mmHg + 355 mmHg = 500 mmHg
V = 3.0 L + 2.0 L = 5.0 L


The partial pressures would be
He = 3.0 L ; Ar = 2.0 L

Right?

But the thing is, in order to find the partial pressure we would need to know the number of moles each gas has, then the mole fraction. Now knowing the total pressure we can use the mole fraction to find the partial pressure of each of the gases, correct?

BUT without a known temperature, how do we find the number of moles for the gases since we can't use PV=nRT ??


Someone suggested using Boyle's Law P1V1=P2V2 . . .  But that doesn't make sense to me. . .

Help??

[AWK]

Someone suggested using Boyle's Law P1V1=P2V2 . . .  But that doesn't make sense to me. . .

This make sense (V2 = 5L)

[Borek]

Total pressure = 145 mmHg + 355 mmHg = 500 mmHg

That's wrong.

V = 3.0 L + 2.0 L = 5.0 L

That's OK.

You may treat each gas separately when calculating its pressure. Later just sum the pressures up.

[Norngpinky]

Ah, ok, that makes more sense...

So...

Because 145mmHg and 355mmHg are pressures of the individual gases before they merged, these values are not partial pressures...And because the new volume would be enlarged, and therefore, with the same values of moles from the two gases, the partial pressures will be less... Right?

And thus, like AWK suggested...

P1V1 = P2V2 (1=He, 2=after)
P2 would be the partial pressure of He after the valve is open.

do the same for Ar, and add the partial pressure up to get the total pressure, which isn't 500mmHg.

Is that correct reasoning?

[Borek]

Seems OK to me

[Norngpinky]

Ah, thank you! ^__^