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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: Aakash on June 15, 2013, 08:26:54 AM

Title: Root mean square speed
Post by: Aakash on June 15, 2013, 08:26:54 AM

Utter confusion! Please help'!

There is a question in my book where we have to find root mean square velocity of oxygen at STP provided the density of oxygen. I know the formula that is sqrt3RT/M. But my answer is not coming right and in the solution they have used the formula sqrt(Pressure/density).

Title: Re: Root mean square speed
Post by: Hunter2 on June 17, 2013, 06:06:41 AM

Check the units. Only SQR(pressure/density) give a unit for speed.

Title: Re: Root mean square speed
Post by: curiouscat on June 17, 2013, 06:57:27 AM

Quote from: Hunter2 on June 17, 2013, 06:06:41 AM

Check the units. Only SQR(pressure/density) give a unit for speed.

No. Other one does too.

Title: Re: Root mean square speed
Post by: Hunter2 on June 17, 2013, 07:29:53 AM

Yes you are right

Title: Re: Root mean square speed
Post by: Nisarg Shah on June 17, 2013, 09:48:19 AM

Quote from: Aakash on June 15, 2013, 08:26:54 AM

There is a question in my book where we have to find root mean square velocity of oxygen at STP provided the density of oxygen. I know the formula that is sqrt3RT/M. But my answer is not coming right and in the solution they have used the formula sqrt(Pressure/density).

Most popular formula for RMS speed is v_rms=√^3RT/_M. Using the gas equation and density=mass/volume, you can prove that v_rms=√^ρ/_p (try it yourself), where ρ is the density of the gas.
So both of these equations are the same. This means there is something wrong in your calculations.
Most probable error would be that you've not put the values with correct units.
Reply if you're still not getting your answer.

Title: Re: Root mean square speed
Post by: Aakash on June 18, 2013, 12:53:26 AM

Thank You!
You were right.