# How much heat needed?

## Homework Statement

7 kg of ice at 0◦C is converted to steam at 100◦C. How much heat is needed? The heat of fusion for water is 79500 cal/kg, its heat of vaporization is 5.4 × 105 cal/kg, and its speciﬁc heat is 1000 cal/kg ·◦C. Answer in units of kcal.

## Homework Equations

What is the formula that I am supposed to use?

Q=mc$$\Delta$$T ?

You *also* need to use

Q = mL,
where m is the mass and L is the heat of fusion/vaporisation.

R.

you go from solid to liquid to gas, so you have to calculate the heat for each stage than add up the total of each stage. I think, also, for the vaporization you would use Q =ml where L = latent heat of vaporization.. and Q=ml again for latent heat of fusion/melting

I think thats how you do it...

Last edited:
You should end up with 3 separate values for Q which you will have to add up.

So:

Q=mc$$\Delta$$T
Q=(7kg)(1000 cal/kg C)(100 C)
Q= 700,000 cal

heat of fusion:
Q=mL
Q=(7kg)(79500 cal/kg)
Q= 556,500 cal

heat of vaporization:
Q=mL
Q=(7kg)(5.4x105 cal/kg)
Q=3,780,000 cal

$$\sum$$Q= 700,000 + 556,500 + 3,780,000

$$\sum$$Q=5,036,500 cal = 5036.5kcal

Is that how?!

Indeed :)

Thank you!