RMS speed of gas

[ChloeYip]

Homework Statement

(introduction course of university physics)[/B]

A 5.0- liter gas tank holds 1.7 moles of monatomic helium (He) and 1.10 mole of diatomic oxygen

(O2), at a temperature of 260 K. The ATOMIC masses of helium and oxygen are 4.0 g/mol and

16.0 g/mol, respectively. What is the ratio of the root- mean- square (thermal) speed of helium to

that of oxygen?

Answer: C

Homework Equations

V(rms) = sqrt(3kT/m)

The Attempt at a Solution

This is the only equation I found from my book about rms speed of gas, but the variables seems unrelated at all. I have no idea on how to do it.

Please tell me how to deal with it in a more detail way.
Thank you very much.

 

[DrClaude]

1
This is the only equation I found from my book about rms speed of gas, but the variables seems unrelated at all.

Unrelated to what? This is actually the correct equation to use.

 

[ChloeYip]

but how to pluck in the numbers? what should i manipulate to get the required variables?
can you be more specific please?
thanks

 

[DrClaude]

The question is

What is the ratio of the root- mean- square (thermal) speed of helium to
that of oxygen?

Start with that. How do you write that ratio?

but how to pluck in the numbers?

What are k, T, and m?

 

[ChloeYip]

How do you write that ratio?

just divide the two which substituded in the equation

of course I know I have to find out all the variables mentioned in the question and the equation in order to answer the question...
but I just have no idea how to figure them out...
can you be more specific? i can't even guess them out... please don't just simply ask me to calculate the answer, because that's exactly my problem for can't getting the answer!

What are k, T, and m?

k is Boltzmann constant
t is temperature
m is mass of a molecule
of course i know them, by just substitute them!, but how to find them out?

sqrt(3*1.38e-23*260/1)/sqrt((3*1.38e-23*260/16)=4
but the answer is 2.8

I REALLY have no idea
whats wrong by plucking them in
or if i did something totally in the wrong track

please be more specific on how to do!
like how you figure out the answer
or your steps to calculate it
i can't do anything further based on your question which that seems to be common sense

thank you very much

 

[ehild]

k is Boltzmann constant
t is temperature
m is mass of a molecule
of course i know them, by just substitute them!, but how to find them out?

sqrt(3*1.38e-23*260/1)/sqrt((3*1.38e-23*260/16)=4

What are the masses of the molecules? The atomic mass of He is not 1 and the molar mass of the O₂ is not 16.

 

[ChloeYip]

oh yes thanks