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Question:

When an ideal monoatomic gas is heated at constant pressure, the fraction of the heat energy supplied which increases the internal energy of the gas is

3/5

3/7

5/7

2/5

Solution:

By first law of thermodynamics
Q=U+W=f2nRT+PΔV=f2nRT+PV
or
Q=f2nRT+nRT(∵PV=nRT)=32nRT+nRT=52nRT
∴Fraction of heat energy supplied=UQ=(3/2)nRT(5/2)nRT=35.