[mslee]

Hi,

I think I'm confused over something I shouldn't be confused about Is the mass of hydrogen molecule just the mass of 2 electrons? Are the 2 protons' mass negligible?

Also, when do you use the reduced mass as opposed to what I just said? Because a hydrogen molecule IS a 2 atom system, so should I use the reduced mass? (the question asks for velocity of hydrogen molecule after absorption of photon of a given wavelength (H2 is originally at rest and all momentum is assumed to be conserved in collision).. so I was going to do h/lambda_photon=m_H2v_H2... does that sound right? I'm just not sure what to use for "m"... And then for the second part of the question, the translation energy of H2 molecule after collision would be just 1/2mv²?

[enahs]

I think I'm confused over something I shouldn't be confused about Is the mass of hydrogen molecule just the mass of 2 electrons? Are the 2 protons' mass negligible?

First, lets start here.

Look up the mass of protons and mass of electrons.

Which one is more massive and which one is negligible?

[mslee]

Well, okay, so mass of a proton is bigger by like 4 orders of magnitude, so negligible.

[enahs]

Well, okay, so mass of a proton is bigger by like 4 orders of magnitude, so negligible.

So, say I have object A that weighs 1 grams, and object B that weighs 10,000 grams (4 orders of magnitude bigger).

The mass of object A + B = 10,000g + 00001g = 10,001g.

So percent wise, object A accounts for ( (1/10,001)*100 ) = 0.00999 percent of the total mass.
Percent wise, object B accounts for ( 10,000/10,001) * 100 ) = 99.99 percent of the total mass.

Which one is negligible?

[mslee]

Yes, I get that the mass of e- is negligible now. So in the equation I'm using for the problem, would I use the reduced mass for 2 protons for "m" or would I use the sum of 2 protons' mass for "m"?

[Yggdrasil]

For calculation of the translational kinetic energy of the hydrogen after the collision, you would use the mass (mass of two protons).  The main situation where you use reduced masses is for calculating vibrational or rotational kinetic energy.