[Khemikooligan]

I need help with a problem on the practice exam for my Pchem test next week. My professor has posted what the right answers are but he has not shown the full solutions for them. Here is the problem:

Image Link: https://i.imgur.com/p44IHci.png




The Δ_vapH for the reaction is given on the last page, it is 51.6 kJ/mol

My thinking for the problem was that since pressure is constant, work is 0, hence ΔU = q, and since ΔH = q at costant pressure, ΔU = ΔH. So since H₂O₂ is condensing, then ΔH = -51.6 kJ/mol which would also be the value for ΔU.

However, the answer my professor gave for the problem was -48 kJ/mol. I was wondering if anyone could tell me what I have done wrong?

[mjc123]

Why do you assume no work if pressure is constant? Work = PΔV.