Question Calculating most probable speed ( Physics Forums Advanced Physics ) Updated: 20081219 05:20:17 (7) 

Calculating most probable speed
1. The problem statement, all variables and given/known data
Diatomic carbon dioxide gas (CO2 has molar mass of 44.0 g/mol) is at a temperature of 305 K.
Calculate the most probable speed v_mp.
2. Relevant equations
v_mp = sqrt(2kT/m)
3. The attempt at a solution
v_mp = sqrt(2 * 1.3801 * 10^23 * 305 / 44)
v_mp = 1.383 * 10^11


Answers: Calculating most probable speed ( Physics Forums Advanced Physics ) 

Calculating most probable speed
Looks like molecules should be moving a little faster than that. Check your math. The approach is correct, however.
naresh


Calculating most probable speed
Got it. The answer was 339 m/s. Thanks!
6021023


Calculating most probable speed
heh, well particles isn't really a physical unit (neither is mol). So what you really have is sqrt(m^2/s^2).
naresh


Calculating most probable speed
Then
v_mp = sqrt(2 * 1.3801 * 10^23 (m^2 * kg / (s^2 * K) * 305 K / (44 g/mol / (6.02E23 particles/mol) * kg/1000g)
This ends up with the units being sqrt(m^2/s^2 * particles)
Is that correct?
6021023


Calculating most probable speed
well, it's a number...
6.023E23 particles/mole
naresh


Calculating most probable speed
I can see that m is not simply 44 mol. It needs to be in kg. m = M/N_A. The units for M is g/mol. What are the units for N_A, Avogadro's number?
6021023


Calculating most probable speed
How about you write down the units of all the numerical values you substituted? It is always good practice to do this, and cancel out the units.
naresh


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