How many calories of heat are released when 400g of steam at 145 degree C is coo

Question

How many calories of heat are released when 400g of steam at 145 degree C is cooled to water at 50 degree C a. 6630 cal b. 28, 640 cal c. 244, 640 cal d. 202,000 cal Due to he normal body temperature of humans, the primary radiation emitted by humans is in which part of the electromagnetic spectrum? a. Gamma rays b. Radio waves c. Infrared d. Ultraviolet Imagine an open container with a block of ice starting at a temperature of -50 degree C. Heat is added continuously the container until all the ice has melted and the water is boiling vigorously. Circle the graph below that best represents the temperature versus time graph for this situation.

Explanation / Answer

To find the heat released when 400 g of steam at 145 C is cooled to water at 50 C requires the heat of vaporization of water that is 2257 kJ/kg, and the specific heat of steam that is 1.996 kJ/kg*K, the specific heat of water that is 4.187 kj/kg.k

When steam at 145 C is cooled first the temperature drops to 100 C, at this temperature there is a change in phase from vapor to liquid, the water then cools till a temperature 50 C is reached.

The total heat released is (400/1000)(45*1.996 + 2257 + 50*4.187) = 1022.5 kJ or 244.43 kCal. (Option c is correct)

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