The pressure of Cl2 gas in a 2.0-L cylinder is 300 kPa at 25°C. What is the pressure (in kPa) when this gas is released into a 100-L chamber at 25°C?

Use the Ideal Gas Law: PV=nRT. Since the values of n, R and T are all constant in this problem, you know that the product of pressure times volume must be constant as well. Thus P_1V_1 = P_2V_2 . You have values for P_1,\ V_1 and V_2, so you can calculate P_2.

If you know Boyle's Law, this would give you directly the formula:

P_1V_1 = P_2V_2

which brings down to the same thing as what ebaines said. :)