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**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?