# Homework Help: Heat that are needed

1. Nov 7, 2007

### jack1234

How many calories of heat are required to raise the temperature of 5 kg of water from 60° to the boiling point? Select the correct answer.

a. 4.2 x 10^5
b. 2 x 10^5
c. 7.5 x 10^4
d. 18 x 10^5
e. 420

The answer is a, but what I calculate
Q=mcdeltaT
=5*4186*1
=20930
is b, am I doing anything wrong?

2. Nov 7, 2007

### CompuChip

What is the boiling point of water?
What is Delta T ?

3. Nov 7, 2007

### malty

Em, i suppose you mean $$Q=5*4186*10$$ and in that you included your division by four to change the joules into calories.

I can't see anything wrong anyways, unless it wanted you to include the latent heat of vaporization into the formula, though I doubt that would change your answer by much.

I may be wrong too though..

Last edited: Nov 8, 2007
4. Nov 7, 2007

### jack1234

O...I think the answer in original question is not correct, it should be
it should be
Q=5000g * 1 J/(calories*celcius) * 40 celcius
=20000J

However the answer is still b though, what is the problem?

5. Nov 8, 2007

### CompuChip

You're quite close, but I have the feeling you're mixing up your units.
Here's the correct calculation (note how I leave in the units and let them cancel at the end).

Q = m * C * dT
= 5 kg * 4186 J/(kg C) * 40 C
= 837,200 J.

Now taking 1 cal = 4.184 J (which seems to be the accepted value),
Q = 200,095 cal = 2 x 105
in the correct number of significant digits.
So the answer is b) -- if it says it's a) then it is wrong