Posted by **Hannah** on Wednesday, January 18, 2012 at 8:27pm.

Which molecule below has the greatest average velocity at 100 degrees celsius?

He, Cl2, Xe, O2, NO2

I know that I already posted this the other day and the response I was given was to use the root mean squared equation. I know that the molecule with the smallest mm will be the fastest and I used the root mean squared equation but I think I did it wrong. The answer I chose was Xe as the greatest because I came up with 0.0076 but now I am confused and do not think I did this right. The regular mm of Xe is the biggest number out of all the answers so then Xe would not be the answer.

Could someone please just make sure I did this correct? Thank you!!

- Chemistry -
**DrBob222**, Wednesday, January 18, 2012 at 9:40pm
I gave you the rms speed earlier; I thought that might be easier to handle but the answer you get should be the same as the kinetic energy calculation. The average kinetic energy of a molecule is 1/2 mv^2 and the average KE of different molecules is the same at the same T.

So if KE is the same for all of the molecules, then

KE = 1/2 mv^2 where v is the average speed of the molecules.

If we solve for v, we obtain v = sqrt(2*KE/m).

Look at the equation. m is in the denominator which means v is indirectly proportional to m. So the heaviest molecule will have the least average velocity; the lightest molecule will have the highest average speed. Which is the heaviest molecule? which is the lightest molecule? You don't need to do any calculations. T is a constant for all of these, the only variable is molar mass so v = k*sqrt(1/M).

- Chemistry -
**Hannah**, Wednesday, January 18, 2012 at 9:51pm
Is K the temp in kelvins?

So for an example He would be

373K(sqrt 1/4.0026) = 186.43

Did I do that right?

- Chemistry -
**DrBob222**, Wednesday, January 18, 2012 at 10:15pm
I don't know to what you refer. K in KE stands for kinetic energy. Don't obsess about this. rms speed He = sqrt(3RT/M) = sqrt(3*8.314*373/4) = ?

rms speed Xe = sqrt(3*8.314*373/131.3) = ?

The others are done the same way and you simply pick out the greatest rms speed. That isn't the average speed but it will do. For average speed one uses the kinetic energy.

However, note that you do NOT need to do any calculations.

If kinetic energy = KE = 1/2 mv^2, then

v = average speed = sqrt(2*KE/mass)

so highest mass = lowest speed

lowest mass = highest speed. That's all there is to it.

- Chemistry -
**Hannah**, Wednesday, January 18, 2012 at 10:28pm
ok so the lowest mass is Xe and the highest mass is NO2.

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