# Homework Help: Calculate the heat needed to change 2kg of ice to steam

1. Nov 29, 2008

### honkydora5

1)Calculate the heat needed to change 2kg of ice at 0 degrees C to steam at 100 degrees C.

The attempt at a solution
2x334000=66800J
2x4200x100=840000J
2x2260000=4520000J
total=5426800J should be 6120000J somehow

2)An electric kettle produces 3000J of heat every second.It is filled with water ,weighed and switched on. After coming to the boil,it is left on for a further 80 second and then switched off.It is found to be 100g lighter.Calculate the specific latent heat of steam.

Thank for help!!!!!

2. Nov 29, 2008

### malawi_glenn

Re: Heat

Latent heat of melting is 333J per gram
Heat capacity of water is 4.18J per gram and Kelvin
Latent heat of vapourization is 2260 per gram

So in total I get: 6146000 which is much closer to the correct answer 6120000J

It just depends what table you are using.

For your second question, you need to show some work. Hint: What is the definition of Latent Heat.

Also, what has this to do with Advanced physics? Sounds like introductory thermodynamics according to me.

3. Nov 29, 2008

### honkydora5

Re: Heat

That sounds good to me ,but can you show some calculations?How did you get 6146000
P.S I know that it is basic physics but im stuck.
Thanks for help anyway.

4. Nov 29, 2008

### malawi_glenn

Re: Heat

by using the figures I just posted??? what else?

Who are YOU to ask ME for calculations? This is charity..

5. Nov 29, 2008

### honkydora5

Re: Heat

By using those calculations i get 5426800.so its not much help is it?

6. Nov 29, 2008

### malawi_glenn

Re: Heat

Then maybe you need to practive using the calculator properly.

I got 6022000 now LOL

Can you SHOW you calculations perhaps?

As I said, it is just a matter of having the same table as the person who constructed the answer. Dont bother so much about numbers. The important thing to know is to use the latent heat and specific heat.

By the way, I got 6028000 using the figures you had in your first post.

7. Nov 29, 2008

### honkydora5

Re: Heat

thanks for spoting mistake!!!!!!!!!!!!!!!!!!!! i missed one zero . uuuufffff.so im not that bad???????
Any chance for help with second one?
The specific latent heat of vaporisation of substence is the amount of energy needed to boil the liquid to gas without changing the temperature.
And again I got 240J/kg should be 2400000

8. Nov 29, 2008

### malawi_glenn

Re: Heat

Calculate the specific latent heat of steam.

Now careful, you said that answer is 2400000, same units as you?!?! Sometimes one forget to compare units..

The amount of energy is 3000*80J
The mass is 100g, which is 2400J/g = 2400 000J/kg !

9. Nov 29, 2008

### honkydora5

Re: Heat

was it that simple? I think I will waste half of my life making mistakes! ;-(((
Thanks for your time and help!!!!!

10. Nov 29, 2008

### malawi_glenn

Re: Heat

Sometimes it is really simple :)