Answer:
n = 1.89 mol.
Explanation:
Think of this as adiabatic free expansion. This means constant temperature -- we know this because the internal energy does not change.
Since the temperature is constant, the entropy change can be given by:
Entropy = nRln(V2/V1)
Entropy = 17.28 J/K. R = 8.314 (gas constant). V2 = 3V. V1 = V.
Thus, we get:
17.28 = n(8.314)ln3
Solve for n. We get n = 1.89 mol.