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NEET PHYSICSMedium

4.0 gm4.0 \text{ gm} of gas occupies 22.4 litres22.4 \text{ litres} at NTP. The specific heat capacity of the gas at a constant volume is 5.0 J K1mol15.0 \text{ J K}^{-1}\text{mol}^{-1}. If the speed of sound in the gas at NTP is 952 ms1952 \text{ ms}^{-1}, then the molar heat capacity at constant pressure will be: (R=8.31 J K1mol1R=8.31 \text{ J K}^{-1}\text{mol}^{-1})

A

8.0 J K1mol18.0 \text{ J K}^{-1}\text{mol}^{-1}

B

7.5 J K1mol17.5 \text{ J K}^{-1}\text{mol}^{-1}

C

7.0 J K1mol17.0 \text{ J K}^{-1}\text{mol}^{-1}

D

8.5 J K1mol18.5 \text{ J K}^{-1}\text{mol}^{-1}

Step-by-Step Solution

At NTP (Normal Temperature and Pressure), 22.4 L22.4 \text{ L} of an ideal gas corresponds to 1 mole1 \text{ mole}. Therefore, the molar mass of the gas is M=4.0 g/mol=4.0×103 kg/molM = 4.0 \text{ g/mol} = 4.0 \times 10^{-3} \text{ kg/mol}. The speed of sound in a gas is given by the formula: v=γRTMv = \sqrt{\frac{\gamma RT}{M}} Squaring both sides and solving for γ\gamma: γ=v2MRT\gamma = \frac{v^2 M}{RT} Given v=952 ms1v = 952 \text{ ms}^{-1}, R=8.31 J K1mol1R = 8.31 \text{ J K}^{-1}\text{mol}^{-1}, and T=273 KT = 273 \text{ K}: γ=(952)2×4.0×1038.31×273=906304×0.0042268.631.6\gamma = \frac{(952)^2 \times 4.0 \times 10^{-3}}{8.31 \times 273} = \frac{906304 \times 0.004}{2268.63} \approx 1.6 Alternatively, using density ρ=MassVolume=4.0×103 kg22.4×103 m3\rho = \frac{\text{Mass}}{\text{Volume}} = \frac{4.0 \times 10^{-3} \text{ kg}}{22.4 \times 10^{-3} \text{ m}^3} and pressure P=1.013×105 PaP = 1.013 \times 10^5 \text{ Pa}: γ=v2ρP=(952)2×(4.022.4)1.013×1051.6\gamma = \frac{v^2 \rho}{P} = \frac{(952)^2 \times \left(\frac{4.0}{22.4}\right)}{1.013 \times 10^5} \approx 1.6 The ratio of molar heat capacities is γ=CpCv\gamma = \frac{C_p}{C_v}. Given the molar heat capacity at constant volume Cv=5.0 J K1mol1C_v = 5.0 \text{ J K}^{-1}\text{mol}^{-1}: Cp=γCv=1.6×5.0=8.0 J K1mol1C_p = \gamma C_v = 1.6 \times 5.0 = 8.0 \text{ J K}^{-1}\text{mol}^{-1}

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