A) A 1.0 kg block of Copper at 20 degrees Celsius is dunked into a large bucket

Question

A) A 1.0 kg block of Copper at 20 degrees Celsius is dunked into a large bucket containing liquid nitrigen at 77.3 K. 77.3 K is the boiling point of nitrogen. The bucket is insulating enough that we will consider the combined copper-nitrogen system to be isolated. How much mass of liquid nitrogen boils away by the time the copper reaches 77.3K? (Cm,copper= 0.092 cal/g degrees celsius and Lnitrogen, vaporization=48.0 cal/g)

B) for the nitrogen that does boil, what is the average velocity of the molecules just as they enter the gas state, approximating nitrogen gas as ideal? (nitrogen has 0.029 kg/mol

Explanation / Answer

1000g block of Cu cools from 293K to 77.3K, that is 215.7 K,

so it loses 215.7x1000x0.0920=19936.4 cals of heat.

The N is at its BP, so all this heat goes into turning it from liquid to vapour, so 19936.4/48=413.425 g boils away.

2) Vavg = sqrt(8RT/piM)

T = 77.3K , R = 8.314 x J/Kmol M = molecular weight

= Vavg = 7.65 m/s

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