4.0 gm of gas occupies 22.4 litres at NTP. The specific heat capacity of the gas at a constant volume is 5.0 J K−1mol−1. If the speed of sound in the gas at NTP is 952 ms−1, then the molar heat capacity at constant pressure will be: (R=8.31 J K−1mol−1)
A
8.0 J K−1mol−1
B
7.5 J K−1mol−1
C
7.0 J K−1mol−1
D
8.5 J K−1mol−1
Step-by-Step Solution
At NTP (Normal Temperature and Pressure), 22.4 L of an ideal gas corresponds to 1 mole.
Therefore, the molar mass of the gas is M=4.0 g/mol=4.0×10−3 kg/mol.
The speed of sound in a gas is given by the formula:
v=MγRT
Squaring both sides and solving for γ:
γ=RTv2M
Given v=952 ms−1, R=8.31 J K−1mol−1, and T=273 K:
γ=8.31×273(952)2×4.0×10−3=2268.63906304×0.004≈1.6
Alternatively, using density ρ=VolumeMass=22.4×10−3 m34.0×10−3 kg and pressure P=1.013×105 Pa:
γ=Pv2ρ=1.013×105(952)2×(22.44.0)≈1.6
The ratio of molar heat capacities is γ=CvCp.
Given the molar heat capacity at constant volume Cv=5.0 J K−1mol−1:
Cp=γCv=1.6×5.0=8.0 J K−1mol−1
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