#7 from Sapling Week 5/6

[Namita Shyam 3G]

When finding DeltaS, why did we do DeltaH/T? Why can't we do q/T (in general, isn't DeltaS=q/T)? For example, for part a, why can't we do q/T=99360J/351.3K?

[IsaacLaw1E]

DeltaS = q/T is used for the entropy of heating something. So if I heated water from 25C to 100C, I would use this equation.
DeltaS = DeltaH/T is used for the entropy of a phase change, including vaporization. So if I turned water at 100C to vapor at 100C, I would use this equation.
In the question, the samples are already heated to their boiling points, so no extra heating is occuring. It's only vaporization, so we use the second equation.

[Sera Aintablian 2E]

The entropy of phase changes is found specifically by the deltaHvap / T relation. Q is a general heat transfer value, not specific to the entropy of vaporization. Also q = deltaH at constant pressure anyway, and that is stated in the question!

[Namita Shyam 3G]

Wow that makes so much more sense!! Thank you so much guys!!