Calculating the Average Kinetic Energy of a Helium Atom at 20°C

[jgens]

Homework Statement

What is the average kinetic energy of an atom of helium gas at a temperature of 20 degrees C?

Homework Equations

N/A

The Attempt at a Solution

Here's my attempt at a solution: Using the equation E = kT/2 for each degree of freedom, I found that KE = 3kT/2 since an atom of He has three degrees of freedom in translational KE. Using this logic I fould that KE = 6.1 x 10^-21 while the correct answer should be 4.0 x 10^-21. Can someone help me understand where my logic/reasoning is incorrect? Thanks.

 

[Delphi51]

Looks okay to me. Someone got the 4.0 answer by just finding kT.

 

[jgens]

That's what I thought my teacher might have done. Thanks!