An adsorption isotherm apparatus consists of a dosing chamber with a volume of 30 milliliters as well as a sample chamber. The two airtight chambers are initially separated by a valve and at a pressure of 0 torr. Helium gas is let into the dosing chamber until the pressure reaches 600 torr. The valve is then opened, and the helium expands into the sample chamber. Once the pressure has equilibrated between both chambers, it is measured to be 400 torr. In milliliters to two significant figures, determine the volume of the sample chamber.