[scrum]

The molar entropy of helium gas at 25°C and 1.00 atm is 126.1 J K-1 mol-1. Assuming ideal behavior, calculate the entropy of the following.

(a) 0.215 mol He(g) at 25°C and a volume of 5.74 L
 J/K
(b) 4.24 mol He(g) at 25°C and a volume of 3442.4 L
 J/K

I looked at this and i figured since 126.1 is J/Mol K and they want J/K all i do is multiply by mols and not use the other stuff for it.

I did part (a) this way, doing .215 * 126.1 = 27.1

and then for part (b) i did the exact same thing 4.24 * 126.1 and got 534.664 which is incorrect. i also tried variations like 534, 534.7 , 534.66, but they are all wrong.

[Yggdrasil]

IIRC, the entropy of the gas will depend on its volume (and hence pressure).  So, in your calculations you should include the ΔS for the isothermal expansion/compression, needed to get the correct volume/pressure.