# Homework Help: Kinetic theory of gases theory question

1. Oct 23, 2009

### Mandavi

1. The problem statement, all variables and given/known data
Consider a mixture of oxygen and hydrogen kept at room temperature.As compared to a hydrogen molecule an oxygen molecule hits the wall-
(a) with greater average speed
(b)with smaller average speed
(c)with greater average kinetic energy
(d)with smaller average kinetic energy

2. Relevant equations
We have to consider both the gases as ideal to get the answer.
Now, for a mixture vrms=(3RT/M)^1/2
where M is average mass of the gases in the mixture and is
M=(n1M1+n2M2)/n1+n2
n denote the number of moles of gases and M their respective molecular masses.
Average kinetic energy for a gas molecule=3/2kT
k=boltzmann constant
T=absolute temperature

3. The attempt at a solution
The answer given is (b)with smaller average speed.Someone please explain the reason for this.

2. Oct 23, 2009

### Dweirdo

If I understood it right :
both molecules have the same average internal energy, which is kinetic energy
now,the oxygen molecule is heavier than the hydrogen molecule , thus the velocity of the oxygen molecule should be smaller.
mv^2/2 = E
V=(3RT/M)^1/2 when the mass is higher . velocity is lower...

3. Oct 23, 2009

### Mandavi

Right,but doesn't the word "average" has any significance here?

4. Oct 23, 2009

### Dweirdo

I would rather say Energy per molecule.
the "Average" thing is here because this measures the average velocity ,but it has (if it has) a little effect on the answer. the difference between 2 different types of molecules in this case is big, so you can drop that word.

5. Oct 23, 2009

### Mandavi

Okay!!Thank you very much!