Question

The internal energy of one gram of helium at 100 K and one atmospheric pressure is

• A. 100 J
• B. 1200 J
• C. 300 J
• D. 500 J

Answer 1

Answer: (C)

300 J

Answer 2

The helium molecule is monoatomic and hence its internal energy per molecule is $\frac{3}{2}k_{B}T$ (where $k_{B}$is the Boltzmann, constant). The internal energy per mole is therefore is $\frac{3}{2}RT.$ One gram of helium is one fourth mole and hence its internal energy is $\frac{1}{4} \times \frac{3}{2}R \times 100 = 300J,$ taking the value of R to be approximately 8J $mol^{- 1}K^{- 1}.$