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.