[pistaciolow]

Calculate the total heat (J) needed to convert 0.349 mol gaseous ethanol at 401°C and 1 atm to liquid ethanol at 25.0°C and 1 atm.

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

 q=mcΔT.

So...
.349(1.43)(674-298)=187.65
              +
.349(2.45)(674-298)=321.498
= 509.1488


Correct method? I'm not sure the ΔT is correct when changing from gas to liquid.       

[Borek]

Wrong. You have to cool down gaseous ethanol to its boiling point, condense, then cool liquid down.

[pistaciolow]

So should it be:

q=mcΔT, with ΔT 401°-78.5° C and cgas
then Δhvap x n
q=mcΔT with 25°-78.5°C as ΔT and cliquid

All added together? with T in Kelvin of course.

[Borek]

Ethanol doesn't boil at 100 deg C.

[pistaciolow]

I guess you saw that before I edited. I tried it anyway, but it didn't work.

here's what I tried:
m=.349 mol x 46.7g/mol=16.298 g

q1=16.298g x 1.43 J x 322.5= 7516.23 J
q2 = .349mol x 40.5 kJ x 1000 J = 14134.5 J
q3 = 16.298g x 2.45 J x -53.5 = -2136.26 J
                                              = 19514.47 J
                                              =1.95e4 J

[pistaciolow]

I recalculated the molar mass and realized the tf-t1 for q1 was wrong, but still nothing...

here's what I tried:
m=.349 mol x 46.068 g/mol=16.0777 g
delta T for q1= 78.5-401=-322.5
delta T for q3= 25-78.5=-53.5

q1=16.0777g x 1.43 J x (-322.5)= -7414.633298 J
q2 = .349mol x 40.5 kJ x 1000 J = 14134.5 J
q3 = 16.0777g x 2.45 J x -53.5 = -2107.3845J
                                              = 4612.482 J
                                              =4.61e3 J

[DrCMS]

Going from a gas to a liquid is energy absorbed or released?

[pistaciolow]

I was thinking it needed to be cooled/released, and therefore delta t of q3 is negative. So are you saying it should be positive instead?

[DrCMS]

No, I meant q2. (q3 is cooling a liquid and does not involve any gas)

You start with a hot gas, you cool it down to its boiling point, you condense it to a liquid and then cool the liquid.

Do any or all the step require you to remove energy?

Should all the steps have the same or different sign in front of them?

No look at what you did last and recalculate the correct answer.