A substance has a melting point of 20C and a heat of fusion of 3.9 × 104 J/kg. T

Question

A substance has a melting point of 20C and a heat of fusion of 3.9 × 104 J/kg. The boiling point is 150C and the heat of vapourization is 7.8 × 104 J/kg at standard pressure. The specific heat capacities for solid, liquid and gas are 600 J/kg·K, 1000 J/kg·K, and 400 J/kg·K, respectively. 31) The quantity of heat required to raise the temperature of 3.8 kg of the substance from -6Cto 90Cat 1.0 atm is approximately: A) 3290 kJ B) 325 kJ C) 207 kJ D) 473 kJ Answer: D 32) If 30 kJ of heat is added to 1.2 kg of the substance at 15C, what will be the final temperature? A) 20C B) 57C C) 25C D) 42C

Explanation / Answer

1) Q = m * c *(CHANGE IN TEMPERATURE)

c = specific heat capacity.

From -6 to 90 degrees C

amt of heat required to raise temp of3.8 kg sample from -6C to 90C

= heat from -6 to 20 C + heat of fusion at 20C (3.9 × 10^4 J/kg) +heat from 20 to 90C

= 3.8*600*26+3.8*3.9*10^4+3.8*1000*70

=473480 = 473KJ

OPTION D

2.

If 30 kJ of heat is added to 1.2 kg of the substance at 15C, what will be the final temperature?

Q = m * c *(CHANGE IN TEMPERATURE)

c = specific heat capacity.

Given 30 KJ is added = 30000 J is added.

From 15 to 20 C = 1.2*600*5 = 3600J

Remaining heat is 30000-3600 = 26400 J

heat of fusion at 20 (3.9 × 10^4 J/kg) = 1.2*3.9*10^4 = 46800J

we see heat of fusion is more compared to remaining heat

so the substance will be at 20 C
Option A

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