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A 5 kg substance absorbs heat at a constant 42 kJ/min, and its temperature versus time is graphed. Using the flat portion of the graph corresponding to vaporization (lasting 75 min in the standard data), the latent heat of vaporization of the substance is:

1. 630 kJ kg⁻¹
2. 126 kJ kg⁻¹
3. 84 kJ kg⁻¹
4. 12.6 kJ kg⁻¹

Correct answer: 630 kJ kg⁻¹

Solution

Heat in during the vaporization plateau divided by the mass gives the latent heat; with the standard plateau the value is 630 kJ/kg.

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