# RMS speed of gas

1. Dec 9, 2016

### ChloeYip

1. The problem statement, all variables and given/known data

(introduction course of university physics)

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?

2. Relevant equations
V(rms) = sqrt(3kT/m)

3. 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.

2. Dec 9, 2016

### Staff: Mentor

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

3. Dec 9, 2016

### ChloeYip

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

4. Dec 9, 2016

### Staff: Mentor

The question is

What are k, T, and m?

5. Dec 9, 2016

### ChloeYip

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!

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

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

Last edited: Dec 9, 2016
6. Dec 10, 2016

### ehild

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.

7. Dec 10, 2016

### ChloeYip

oh yes thanks