We have 6 litres (half a bucket) of water at 20 deg C. To raise the temperature to 40 deg C, how much steam at 100 deg C must be added?

200 g
2000/9 g
2 kg
200/3 g

Correct answer: 200 g

Solution

The cold water (6 L = 6000 g) heats from 20 to 40 deg C, absorbing Q = 6000 * 1 * 20 = 120000 cal. Steam of mass m (grams) at 100 deg C first condenses (releasing m*540 cal) then cools from 100 to 40 deg C (releasing m*1*60 cal). Total heat from steam = 600m cal. Setting 600m = 120000 gives m = 200 g.

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