# Average translational kinetic energy of nitrogen molecule

1. Jan 10, 2008

### physics4u

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

2.0 moles of nitrogen (N2) gas are placed in a container whose volume is 8.4 10-3 m3. The pressure of the gas is 4.7 105 Pa. What is the average translational kinetic energy of a nitrogen molecule?

average KEN2 = WebAssign will check your answer for the correct number of significant figures. J

2. Relevant equations

3. The attempt at a solution

2. Jan 10, 2008

### Shooting Star

No equations, no attempt at solving?

3. Jan 10, 2008

### physics4u

Attempt

2.0 moles of nitrogen (N2) gas are placed in a container whose volume is 8.4e-3 m3. The pressure of the gas is 4.7e5 Pa. What is the average translational kinetic energy of a nitrogen molecule?

So I used PV=nRT and solved for T, after solving for Temperature, I used KE=3/2kT and solved for the total kinetic energy. Now is this my final answer or are there more steps to this problem?

4. Jan 10, 2008

### olgranpappy

the *total* kinetic energy due to translational motion is 3/2 NkT

you have solved for the kinetic energy of a typical molecule. which is what you want.

5. Jan 11, 2008

### Shooting Star

For a diatomic gas, you should use 5/2 KT.

6. Jan 11, 2008

### olgranpappy

mmm... no, he/she used PV=nRT to solve for the temperature which holds for any kind of ideal gas, be it composed of molecules or not... cf. Landau + Lif****z Stat Mech Volume I, section 42. then the translational kinetic energy is just 3/2 kT. cheers.

7. Jan 12, 2008

### Shooting Star

Quite right upto the point of finding T. But after finding T, he/she has used E_av = 3/2 KT, which will be true for point masses with only 3 degrees of freedom. The rotational energy is being neglected. So, E_av = 5/2 KT will give the correct answer.

8. Jan 12, 2008

### olgranpappy

but the question in the original post asks only for the average *translational* kinetic energy.

9. Jan 12, 2008

### Shooting Star

OK, I missed the "translational". In that case, it should be 3/2 KT. Thanks.

10. Jan 12, 2011