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Ice at 0 deg C is added in stages to 200 g of water initially at 70 deg C in a vacuum flask. After 50 g of ice has been added and fully melted, the temperature is 40 deg C; after a further 80 g of ice has been added and melted, the temperature is 10 deg C. Taking the specific heat of water as 1 cal/g deg C, find the specific latent heat of fusion of ice.

1. 3.8 x 10⁵ J/kg
2. 1.2 x 10⁵ J/kg
3. 2.4 x 10⁵ J/kg
4. 3.0 x 10⁵ J/kg

Correct answer: 3.8 x 10⁵ J/kg

Solution

Two heat balances (including the flask heat capacity C) subtracted give 30L = 2700 cal, so L = 90 cal/g = 90 * 4186 J/kg approx 3.8 x 10⁵ J/kg.

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