You have a 1 l container of a gas at 20 c and 1 atm. without opening the container, how could you tell whether the gas is chlorine or fluorine?
a. measure the mass of the gas
b. measure the speed of the gas molecules
c. measure the volume of the container
d. measure the pressure of the gas in the container

Without opening the container, a. measure the mass of the gas. Once you get the mass of the gas, you can calculate its density because you already know its volume which has a value of 1 liter. To take the mass of the gas, make sure you don't include the mass of the container. It would be better to measure the container before putting the gas and after putting the gas, and then subtracting the values.