[oxygeniscoool]

Dihydrogen has Δ_vapH_m = 448,69 kJ/kg. How to convert this to unit kJ mol-1 when pressure is 1 atm and T=20,37 K?
I have tried to do this: 448,69 kJ/kg = 448 J/g = 448,69*2,02 = 906 J/mol, but the answer is supposed to be 897 J/mol. So what am I doing wrong?

[Mitch]

Your answer seems right enough, I'm not sure why they gave you pressure and temperature though.

[sjb]

A previous rounding error, perhaps? (note 448.69 x 2 is closer to the given figure). As to why p and T are given, sometimes it's for future use (are you for instance later asked the volume?) or a skill in separating what's required and what's not?

[oxygeniscoool]

Your answer seems right enough, I'm not sure why they gave you pressure and temperature though.

Two densities are given, H2(g) = 0,09 g/dm3 and H2(l)=71 g/dm3. However, it's not sure that this is necessary for the task. Temperature 20,37 K is gas and using Clausius Clapryron-relation the pressure needed to liquefy H2(gas) is supposed to be calc. But the problem is I dont know how to convert the heat of vap.

[Enthalpy]

My data tells 454,3kJ/kg=916J/mol at 1atm hence around 20K.
I don't see how to get 897J/mol even with the given data, except that the book or professor took 2.00g/mol, which is nonsense if the other numbers have 4 or 5 places. 2.016g/mol would be better.

Densities: to convert an enthalpy in an enthalpy, you need no volume nor correction. It's only a matter of unit change. If for instance the temperature differed from the measure's one, you would need some corrections, but usually the necessary data is an enthalpy or a C_p, which includes already the effects of the volumes.