# Latent Heat Question

#### veronicak5678

1. Homework Statement

A well insulated cup contains 125 g of water at 23 degrees. How many grams of ice must be added to lower temp to 12?

2. Homework Equations

Q = c*m*delta t

3. The Attempt at a Solution

energy needed to lower temp:
Q = 4190 J / kg * (-11 degrees) * (.125 kg) = -5760 J

Q from ice = - Q from water = 5760 J

to find mass of ice needed:

5760 J = Latent heat of ice * mass needed = (3.34 * 10^5 J / kg) * m

= .0172 kg = 17.2 g

The correct answer is 15 g. What am I doing wrong?

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#### tiny-tim

Homework Helper
Hi veronicak5678!
A well insulated cup contains 125 g of water at 23 degrees. How many grams of ice must be added to lower temp to 12?

The correct answer is 15 g. What am I doing wrong?
hmm … you didn't stir!

#### veronicak5678

I'm sorry, but I don't follow...
Should I be using something other than latent heat?

#### tiny-tim

Homework Helper
yes … the ice will become water, and mix with the water already there

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