[memfield]

Hello everyone,

My name is Merissa and I am reviewing for a final exam this morning.  The review contains over 60 questions, and I am really struggling with this one.  The answers are already given to us, but that does not do me a a lot of good, as I really need a good understanding of the process to solve.  Below is the question along with the provided information and answer.  From my understanding of the question below, I am solving for grams of steam @ 100°C to raise the temperature of 350g of water surrounding a flask from 25°C to 70°C.  Any help you can offer would be of great help.


A flask was immersed in 350 g of water at 25°C .  Steam at 100°C was passed into this flask , and eventually condensed into water at 100°C.  If the temperature of the water surrounding the flask was raised to 70°C, how many grams of steam must have condensed? (Assume that the condensed water remains at 100°C).

CH2O = 4.184 J/g°C
ΔHvap = 40.7kj/mol

Answer = 29 grams


Thanks again for any help you guys can provide,

Merissa.

[Borek]

What is a heat balance for the system? Where does the heat comes from, where is it going? In other words: what gives the heat, what absorbs it?

[memfield]

This is all of the information that I am given.  Now we are back to assumptions, but I assume, the heat of the steam is a perfect 100°C at all times, and is introduced as the heat source to the system.  The heat is being transferred to the surrounding 350g to make a change from 25°C to 70°C. 

[Borek]

You are still missing something. If the temperature of steam is 100°C all the time, there should be no heating of the water surrounding the flask.

[memfield]

A flask was immersed in 350 g of water at 25°C .  Steam at 100°C was passed into this flask , and eventually condensed into water at 100°C.  If the temperature of the water surrounding the flask was raised to 70°C, how many grams of steam must have condensed? (Assume that the condensed water remains at 100°C).

CH2O = 4.184 J/g°C
ΔHvap = 40.7kj/mol

Answer = 29 grams

Again, this is all of the information provided.  This only being one question on the exam review it is not a huge deal for me to know simply to pass my test, I just really get frustrated when I do not understand the process of solving.  This is the first time anything like this has been asked, and I have no previous work to review that is even close to this.

If the question is missing information, I don't know what I can do about that...

[memfield]

Solved, but I have a problem with this approach

First find the heat energy absorbed by the water.

Joules = 4.18 joules/gm water times 350 gms water times 45 degree C temp increase =  65.835 KJ

Moles steam condensed = 65.835 Kj over 40.7 Kj/mole = 1.618 moles steam.

1.618 moles steam(H20) times l8 gms/mole = 29.12 gms steam condensed.


Even though I received the answer using this approach, I am not satisfied.  There is no account for any efficiency of transfer through the flask.  You will never receive a 1:1 transfer, and I believe this is a pointless exercise without yielding a value that is useful in the real world.

Anyway.  Thanks for looking....

[Borek]

I suppose you misunderstood my questions. I asked you to describe what is going on to guide you to the solution - understanding what is going on is half of the solution. But instead of describing the system - steam is condensing giving off a heat that increases the water temperature, so the heat balance is

m_steam × L = m_water × q × ΔT

you first stated that this information is not given in the question, then you used it to solve the problem.

Your solution is perfectly correct. Energy is conserved, so there is no such thing as efficiency of transfer - all the energy has to be transferred, it can't go anywhere else. Question doesn't ask nor doesn't tell anything about the heat transfer speed - that would depend on the flask, but it is irrelevant to the problem.

[memfield]

I understand the "Teach a man to fish" approach.  It absolutely got me to break down in my head what was happening.  Thanks for that.  I am glad I found the forum, and will more than likely need to revisit often.

Thanks again for taking so much time to respond.