# A levels rms speed problem

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#1
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
0
1 month ago
#2
Have you tried this formula ?√ (3KT)/m
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#3
(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.
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1 month ago
#4
(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)
Last edited by golgiapparatus31; 1 month ago
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#5
(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)
Thanks a lot for making me understand !
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1 month ago
#6
(Original post by tahmidbro)
Thanks a lot for making me understand !
Happy to help

I think this explains the other question you put earlier. I think they took the molar mass of H2 as 1 instead of 2.
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#7
(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.
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