[dodoblaster]

2.0 moles of an ideal gas with Cv = 3R/2, initially confined to a container of volume 5 L and at a temperature 24 C, expand (or are compressed) against a constant external pressure of 0.8 atm until the final pressure of the gas is equal to the external pressure. During this process the system does 4,894 J of work on the surroundings. Calculate the change in entropy, Delta S, of the gas (in J / K).

Calculated V(final) using w=-pex(V2-V1)              V2 = 65.375 L
Calculated T(final) using PV = nRT                       T2 = 318.67 K
Plugged them in Delta S = n*R*ln(V2/V1) + n*Cv,m*ln(T2/T1)

Got an answer of ~42 but the correct answer is 44.5.

Can anyone figure out my mistake?

[DevaDevil]

when I fill in all your values (which you have done properly), I do get to 44.5 J/K