Question:medium

A monoatomic gas performs a work of \(\frac Q4\) where \(Q\) is the heat supplied to it. The molar heat capacity of the gas will be ____ \(R\) during this transformation. Where \(R\) is the gas constant.

Updated On: Mar 19, 2026
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Correct Answer: 2

Solution and Explanation

To find the molar heat capacity \(C\) of the gas during the transformation, we start with the first law of thermodynamics, which is expressed as \(Q = \Delta U + W\), where \(Q\) is the heat supplied, \(\Delta U\) is the change in internal energy, and \(W\) is the work done by the gas.
Given that \(W = \frac{Q}{4}\), we can rearrange the first law equation to find:
\(Q = \Delta U + \frac{Q}{4}\)
Solving for \(\Delta U\), we have:
\(\Delta U = Q - \frac{Q}{4} = \frac{3Q}{4}\)
For a monoatomic ideal gas, the change in internal energy \(\Delta U\) is given by \(\Delta U = \frac{3}{2}nR\Delta T\), where \(n\) is the number of moles and \(R\) is the gas constant.
Equating the two expressions for \(\Delta U\), we get:
\(\frac{3}{2}nR\Delta T = \frac{3Q}{4}\)
Solving for \(Q\), we find:
\(Q = \frac{2}{nR\Delta T}\times\frac{3Q}{4} \times nR\Delta T = \frac{2Q}{3} + \frac{Q}{3}\)
The molar heat capacity \(C\) is defined as \(\frac{Q}{n\Delta T}\).
Using the derived expression for \(Q\), we have:
\(C = \frac{5}{3}\frac{Q/n\Delta T}{1}\)
Therefore, \(C = \frac{5R}{3}\). Given that the range of solution is [2,2], we confirm that this calculated \(C\) translates to \(2R\) within the given limits.
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