Question

a 4kg of air initially at 295k and 325 kpa, is heated at constent pressure in a poston-cylinder device. the final temperature of the air is 405k. assuming the air behaves as an ideal gas with constant specific heat, determine the amount of heat added during the process.

Answer 1

Basically we have to calculate the amount of work done during heating of gas from 295 K to 405 K.

W = -P∆V

Since the pressure is constant lets find the change in volume of the gas i.e. ∆V

∆V = nRT2/P - nRT1/P i.e. nR(T2-T1)/P

where n is the no of moles,R is constant ,T2 & T1 are final and initial temperature respectively, P is the pressure.

n= 4000/28.96 =138.12 moles. (1 mole of atmospheric air weighs 28.96 grams)

∆V = (138.12*0.00831447*110)/325  ^m3 i.e. 0.388 ^m3

Now, W = -P∆V = -325 kpa*0.388 ^m3 =-126.1 kJ

So , A total of 126.1 kJ has been added during the process.