[Chidori]

The question is Calculate the root mean square velocity of the following gases at 25°C
a) H2 b) O2 c) Xe (M = 131.3)

I used the equation: V=(3RT/M)^1/2

I wasn't sure whether to use the R=0.0821 or 8.314 so I tried them both out and couldn't get the right solutions (as seen below)

Correct answers are meant to be:
a) 1.9 x 103 m/s b) 482 m/s c) 238 m/s

The value I obtained for a) was 6.028 (using R=0.0821) or 60.66 (using R=8.314)

Temperature was put in Kelvin so that should be fine so I don't know where I went wrong. (does the answer have to be converted to m/s somehow?)

[smk67]

What are the units involved here? You want velocity, which is m/s. The units of 0.0821 are L-atm/mol-K. The units of 8.314 are J/mol-K. A Joule is defined by the units of kg-m^2/s^2. Using the RMS velocity equation, which equation has the proper units? Use the 8.314 one:

RMS=((3*8.314 kg-m^2/s^2-mol-K*298 K)/mol weight in kg/mol)^1/2.

Your mistake was using the molecular weight in g/mol rather than kg/mol. If you use kg/mol, you are left with units of m^2/s^2 before taking the square root to reduce the units to the desired m/s. Rule of thumb, always keep track of the units as you work out a problem.

[Chidori]

Ahh, thanks heaps for the *delete me*  It's odd though because I've never ever had to convert the units of Molar Mass before in Chemistry.

[Yggdrasil]

Moral of the story:  always include units when you are doing calculations.