Answer : The change in entropy is, 8.65 J/K
Explanation :
Formula used :

where,
= change in entropy
n = number of moles of gas = 1 mole
R = gas constant = 8.314 J/mol.K
= specific heat capacity at constant volume =
(for monoatomic gas)
= initial volume of gas = 1.3 L
= final volume of gas = 3.1 L
= initial volume of gas =

= final volume of gas =

Now put all the given values in the above formula, we get:



Therefore, the change in entropy is, 8.65 J/K