Gases at same temp.

[Chocolatebear89]

What is NOT true about H2(g) and D2(g) at the same temperature?

A. D2(g) has greater momentum than H2(g)
B. D2(g) and H2(g) have the same kinetic energy
C. H2(g) has greater velocity than D2(g)
D. D2(g) and H2(g) molecules exert the same force when they collide with the inner walls of the effusion tube

I narrowed down to A and D, but not sure where to go from there.

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[Pons Asinorum]

What is NOT true about H2(g) and D2(g) at the same temperature?

A. D2(g) has greater momentum than H2(g)

D. D2(g) and H2(g) molecules exert the same force when they collide with the inner walls of the effusion tube

I narrowed down to A and D, but not sure where to go from there.

Answer: D
Temp is the same, so KE is equal. Set H2 and D2 KE equal, and rearrange. mDvD^2 = mHvH^2 => vH/vD = sqrt(mD)/sqrt(mH)
mD = 2 * mH
=> vH/vD = 1.4
Let's ignore units
vHmH = (1.4 * vD)(1)=1.4*vD
vDmD = vD * (2) = 2*vD => momentum of deuterium molecules is higher.

delta (mv)=Favg * delta(time)
=>higher momentum over equal time = more force.

 

[seraphkz]

Really? I thought A was the answer....
Can you tell us where you found this question?

Because all gas molecules exert the same pressure since mass and velocity have an opposite effect to each other. The higher the mass, the lower the velocity. Therefore they have the same momentum, same force on the wall of the container, and same pressure.

 

[Pons Asinorum]

Really? I thought A was the answer....
Can you tell us where you found this question?

Because all gas molecules exert the same pressure since mass and velocity have an opposite effect to each other. The higher the mass, the lower the velocity. Therefore they have the same momentum, same force on the wall of the container, and same pressure.

It states that they're at the same temperature, so you know that they have equal KE. Two objects with equal KE, but different masses, do not have the same momentum. I'm just making some inferrences on what's given, and assuming that the ideal gas law isn't used for the answer.

 

[seraphkz]

Mmmm. But when two objects have the same KE and different masses, it doesn't mean they have different momentum. Because velocity is inversely proportional to mass, higher the mass, lower the velocity. So the effects cancel out, therefore still yielding the same KE and same momentum. No?

Maybe I'm confusing pressure with momentum...I think you might be right. They have the same pressure, same KE but different momentum.

 

[Rabolisk]

Two things with same kinetic energy but different mass have different momentum, mathematically.

 

[2010premed]

in the explanation they say that

(2) mH2vH2 =

(3) mD2vD2 =

and then they just say that since velocity of H2 is bigger, then the denom. is bigger, and the numerator is equal, the momentum of H2 must be bigger. Just another way of looking at it... How do they expect us to come up with this so fast though?
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[Pons Asinorum]

How do they expect us to come up with this so fast though?

I think this is what they expect you quickly to come up with...

Two things with same kinetic energy but different mass have different momentum, mathematically.

From there it's just deduction to get to the right answer.