[kimi85]

The heat of combustion of 2-propanol at 298.15 K, determined in a bomb calorimeter, is -33.41 kJ/g.  For the combustion of one mole of 2-propanol, determine a. delta U and b. delta H

What I did is to convert one mole  to grams and multiplied it to the heat of combustion but my answer is wrong and I don't know how to relate the two unknowns

the answer is -2008 kJ/ mol
and delta H is -2012 kJ/mol

Thank you very much

[Yggdrasil]

Can you show your work?  I have a feeling you just have some rounding errors (in the calculations, be sure to keep at least 4 significant figures at each step).

As for how to calculate ΔU, think of an equation that relates ΔU and ΔH (hint: you may have learned this equation as the definition of ΔH).

[kimi85]

Okay. Thank you very much.

[kimi85]

I had solved U but for H I can't solved it. I think the formula is H= U + P(deltaV) or is it H = U + (delta n)RT? I thought delta V should be 0 but if I used that my answer would still be wrong.

[Yggdrasil]

In this case, both equations are equivalent.  For a general case, you would use the equation:

ΔH = ΔU + Δ(PV)

In the case of a reaction occuring at fixed temperature and pressure plus the assumptions that all gasses are ideal and solids + liquids have negligible volume, you get

Δ(PV) = P(ΔV) = P Δ(nRT/P) = P (RT/P)Δn = RT(Δn)

so

ΔH = ΔU + RT(Δn)

To find Δn, the change in the number of moles of gas, you need to write out the balanced chemical reaction.

[kimi85]

Okay. Thanks again.