Q 11-12-136JEE MainJEE Main 2019 (10 Apr, Shift 1)Easy
A cylinder with fixed capacity of $67.2$ litre contains helium gas at STP. The amount of heat needed to raise the temperature of the gas by $20^\circ\text{C}$ is: [Given that $R = 8.31\ \text{J mol}^{-1}\text{K}^{-1}$]
Answer: (A) $748\ \text{J}$
At STP one mole occupies $22.4\ \text{L}$, so $n = \dfrac{67.2}{22.4} = 3$. The volume is fixed and helium is monoatomic, $C_V = \tfrac32R$:
$$Q = nC_V\Delta T = 3\times\tfrac32\times8.31\times20 \approx 748\ \text{J}$$
Solution by Sreeraj P, M.Sc Physics