# Kaplan FL question pissin me off

#### thebillsfan

##### Unseasoned Veteran
10+ Year Member
5+ Year Member
What is the relative rate of effusion for a mixture of
two noble gases, Gas A and Gas B, which escape through
the same pinhole?

Kaplan says that "Similarly, the pressure of the gas in the container is the same whether you are considering the gas A component or the gas B component, since they are in a mixture characterized by one pressure."

How can you assume thats true? Thats assuming that theres an equivalent number of moles of gas A and gas B in the container, right? If there were diff numbers of moles then the partial pressure of each would be different, in which case you would HAVE to take the partial pressure into account to find the effusion rate.

#### inaccensa

10+ Year Member
7+ Year Member
What is the relative rate of effusion for a mixture of
two noble gases, Gas A and Gas B, which escape through
the same pinhole?

Kaplan says that "Similarly, the pressure of the gas in the container is the same whether you are considering the gas A component or the gas B component, since they are in a mixture characterized by one pressure."

How can you assume thats true? Thats assuming that theres an equivalent number of moles of gas A and gas B in the container, right? If there were diff numbers of moles then the partial pressure of each would be different, in which case you would HAVE to take the partial pressure into account to find the effusion rate.
I hate to confuse u more, but doesn't the effusion rate depend on the molar mass of the gas in question. Higher mass, lower effusion, so according to kaplan, isn't that the same gas?

OP
T

#### thebillsfan

##### Unseasoned Veteran
10+ Year Member
5+ Year Member
I hate to confuse u more, but doesn't the effusion rate depend on the molar mass of the gas in question. Higher mass, lower effusion, so according to kaplan, isn't that the same gas?
No, it was clearly denoted as two different gases w/ different molar masses. That part's covered at least.

#### RogueUnicorn

##### rawr.
7+ Year Member
No, it was clearly denoted as two different gases w/ different molar masses. That part's covered at least.
pressure of ideal gasses are dependent on temp and # moles as per the ideal gas equation. two ideal gasses in the same chamber, provided that they are present in equimolar amounts, should have the same pressure. thus, the only thing affecting their effusion speed should be their molecular masses.

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#### thebillsfan

##### Unseasoned Veteran
10+ Year Member
5+ Year Member
why? PV=nRT. Volume's the same, temp is the same, but if n is NOT the same then P has to be different. why would/should P be the same if n is different?

#### kentavr

10+ Year Member
I don't have a book, but if you are not missing any indications of equal numbers of moles then their assumption is not correct. If we imagine one molecule of gas A and the rest of gas B it does not violate setup of the question. But clearly, to escape from the chamber for molecule "A" will take some time regardless of its molar mass