In order to determine the change in volume, we may use the equation of state for an ideal gas: PV = nRT
Here, the pressure and temperature of the gas is not changing, therefore, we may simplify the expression to: Volume is proportional to moles V / n = constant
V₁/n₁ = V₂/n₂ V₂ = 3.8 * 0.66 / 0.29 V₂ = 8.65 L
0.66 moles of Ar will occupy 8.65 liters of volume.