2. A compression at a constant pressure of 20 kPa is performed on 9.0 moles of an ideal monatomic gas (Cv-1.5R). The compression reduces the volume
of the gas from 0.26 m3 to 0.12 m3. What is the change in the internal energy of the gas? Let the ideal-gas constant R = 8.314 J/(mol. K).
0 -7.0 kj
O zero
4.2 kJ
O-4.2 k]
O 7.0 kj

- 4.2 kJ

Explanation:

We shall find out the initial temperature of the gas

PV / T = n R

20 X 10³ X .26 / T = 9 X 8.3

T = 69.6 K

Since the pressure of gas is constant

V / T = constant

V₁ / T₁ = V₂ / T₂

.26 / 69.6 = .12 / T₂

T₂ = 32.12 K

gas is cooling so there will be decrease in internal energy

Change in temperature = 69.6 - 32.12

= 37.48 K.

decrease in internal energy

= - n Cv x fall in temperature

= - 9 x 1.5 x 8.31 x 37.48  ( Cv = 1.5 R given )

= - 4.2 kJ .

molecules are free to separate from each other and will take on the shape of the container. molecules move completely at random and high speeds. they vibrate in a fixed position and do not move around much. molecules expand to the edge of their container.