The Student Room Group

A levels rms speed problem

The molar mass of oxygen molecules is 16 g. Assuming oxygen behaves as an ideal gas, what is the root mean square speed of oxygen molecules in a sample kept at 15 degrees celcius?
A 21 m/s
B 150 m/s
C 670 m/s
D 440 000 m/s
Any help would be highly appreciated :-D
Have you tried this formula ?√ (3KT)/m
Reply 2
Original post by ThiagoBrigido
Have you tried this formula ?√ (3KT)/m

I have tried : 1/2m<c^(2) = 3/2 k T
<c> = Root Over [(3/2 x 1.38 x 10^(-23) x (15+273) ) / (1/2 x 16 x 1.66 x 10 ^(-27)) ]
= 670 m/s
But I still don't know why '' 16 x 1.66 x 10 ^(-27) '' is done.
Original post by tahmidbro
I have tried : 1/2m<c^(2) = 3/2 k T
<c> = Root Over [(3/2 x 1.38 x 10^(-23) x (15+273) ) / (1/2 x 16 x 1.66 x 10 ^(-27)) ]
= 670 m/s
But I still don't know why '' 16 x 1.66 x 10 ^(-27) '' is done.

It is because it says the molar mass of oxygen "molecules" is 16g/mol.
1 mol of oxygen "molecules" has mass 16g.
1 mol of oxygen molecules is 6.02x10^23 molecules of oxygen
So 1 molecule of oxygen has mass 16/6.02x10^23 g. Convert to kg and you get the same value they got.

Otherwise, the mass of an oxygen "molecule" is 16u, where 1 u = 1.66*10^-27 kg
So the mass of an oxygen molecule is 16 * 1.66 x 10^-27 kg, which is what they used.

(Note: In reality, molecular oxygen is O2, and the molar mass is 32g/mol. That is why I put molecules in inverted commas)
(edited 3 years ago)
Reply 4
Original post by golgiapparatus31
It is because it says the molar mass of oxygen "molecules" is 16g/mol.
1 mol of oxygen "molecules" has mass 16g.
1 mol of oxygen molecules is 6.02x10^23 molecules of oxygen
So 1 molecule of oxygen has mass 16/6.02x10^23 g. Convert to kg and you get the same value they got.

Otherwise, the mass of an oxygen "molecule" is 16u, where 1 u = 1.66*10^-27 kg
So the mass of an oxygen molecule is 16 * 1.66 x 10^-27 kg, which is what they used.

(Note: In reality, molecular oxygen is O2, and the molar mass is 32g/mol. That is why I put molecules in inverted commas)

:smile: Thanks a lot for making me understand !
Original post by tahmidbro
:smile: Thanks a lot for making me understand !

Happy to help :smile:

I think this explains the other question you put earlier. I think they took the molar mass of H2 as 1 instead of 2.
Reply 6
Original post by golgiapparatus31
I think this explains the other question you put earlier. I think they took the molar mass of H2 as 1 instead of 2.

Yes, I agree with you.

Quick Reply

Latest