# Homework Help: Basic thermodynamic chemistry, heat transfer

1. Nov 17, 2012

### qpham26

1. The problem statement, all variables and given/known data

How many grams of steam at 100 deg. Celcius would be required to raise the temperature of 35.8 g solid benzene from 5.5 deg Celcius to 45.0 deg Celcius?
Assume that heat is only transferred from the steam (not the liquid water) and that the steam/water and benzene are separated by a glass wall and do not mix.

2. Relevant equations
Boiling point of benzene 5.5 C
Δfus of benzen is 9.87 kJ/mol
Specific heat of benzene is 1.63 J/g.C and 4.18 for water
ΔHvap for steam at 100 C is 40.7 kJ/mol

so the total heat of the system Qtotal =0= Qgained+Qlost
or Qgained = -Qlost

Qgained = molesbenzen ×ΔH+mbenzen×Cbenzen×(45-5.5)

Qlost = moleswater×ΔHvap + mwater×C×water×(45-100)

3. The attempt at a solution
So it seem like my equation above are wrong
because m comes out as a negative value, which made no sense.
I am pretty sure I got the heat gained correct.
I just uncertain about the heat lost of the steam.

Last edited: Nov 17, 2012
2. Nov 17, 2012

### Staff: Mentor

Wording is confusing. I guess you have to assume water vapor cooled down to 100 deg C and condensed, but the final temperature of water was 100 deg C.

3. Nov 17, 2012

### qpham26

it is the exact wording from the book, and this is just general chemistry, so I dont think there is any complexity in the problem.

so for the heat lost part, if the final temperature is 100 then the initial is 45?
and still, if it is (100-45) based on the above equation I cannot get m as a positive value???

4. Nov 17, 2012

### Staff: Mentor

Oops, sorry, there is no cooling of the vapor, only condensation.

5. Nov 17, 2012

### qpham26

So what should I do with this problem now?
Is my equation correct?
or should it be Qgained = Qlost instead?

6. Nov 17, 2012

### Staff: Mentor

Just leave the mcΔT part for water.

7. Nov 17, 2012

### qpham26

Should it be there?
even though we take out the mcΔT of water, with the negative sign, the answer still comes out to be negative =(

8. Nov 18, 2012

### Staff: Mentor

This sign thing can be confusing, as it can be done in many ways. What is important is to not mix conventions and stick to one. Sorry, no time to give more elaborate explanation now.

Good thing is usually when you mix conventions you get a nonsense result, which makes it easy to spot something is wrong. Negative mass is a sure sign something went wrong.