# Homework Help: Vaporization and change in internal energy

1. Jan 14, 2010

### songoku

1. The problem statement, all variables and given/known data
Suppose 1 g of water vaporizes isobarically at atmosphere pressure (1.013 x 105 Pa). Its volume in the liquid state is Vi = 1 cm3, and its volume in the vapor state is Vf = 1671 cm3. Find the change in internal energy

2. Relevant equations
$$\Delta U = \Delta Q - \Delta W$$

$$\Delta W = p*\Delta V$$

3. The attempt at a solution
I want to ask about $$\Delta V$$.
$$\Delta V = V_2-V_1$$

V2 = Vf, and what is V1 ? Is it Vi or is it zero because volume of vapor is zero initially?

Thanks

2. Jan 15, 2010

### tiny-tim

Hi songoku!

(have a del: ∆ )
It's Vi.

3. Jan 15, 2010

### songoku

Re: thermodynamics

Hi tiny-tim!

Why is it not zero? I think we have to consider the vapor state, excluding the liquid state. Does the first law of thermodynamics only apply for gas?

Thanks

4. Jan 15, 2010

### Redbelly98

Staff Emeritus
Re: thermodynamics

The first law of thermodynamics applies to everything, not just gasses.

5. Jan 15, 2010

### songoku

Re: thermodynamics

I encountered a very similar question.

Consider 100 g (100 cm3) of a liquid evaporating at constant pressure of 100 kPa to vapor of volume 0.167 m3. Assuming that the latent heat of vaporization of the liquid is 2.26 MJ kg-1 K-1 and the vapor behaves like an ideal gas, find the change internal energy.

On the manual, it is written :

note that the volume of gas is zero initially, so ∆V = 0.167 - 0

Is the manual wrong or am I missing something?

Thanks

6. Jan 16, 2010

### Redbelly98

Staff Emeritus
Re: thermodynamics

That's weird.

It makes very little difference in the result, but I was pretty sure you should account for the original volume of the liquid -- if the accuracy of the given numbers warrants it.

7. Jan 16, 2010

### songoku

Re: thermodynamics

ok

Thanks for your help, tiny-tim and Redbelly98 !!