From data below, calculate the total heat (J) needed to convert 0.757 mol of gaseous ethanol (C2H6O) at 151°C and 1 atm to liquid ethanol at 25.0°C and 1 atm.

Question

Boiling point at 1 atm 78.5°C
cgas 1.43 J/g·°C
cliquid 2.45 J/g·°C
ΔH°vap 40.5 kJ/mol

Answer 1

It's easier to work these problems in steps. There are two types of steps you use.
For phase changes (solid to liquid or liquid to gas OR the reverse) you use
q = mass x delta H.
delta H is heat fusion for solid to liquid or reverse.
delta H is heat vaporization for liquid to gas or reverse.

The second type is for heating/cooling within a phase; i.e., heating a liquid from 20 C to 100 C or cooling a liquid from 100 c to 20 C.