You use the ideal gas law equation, which looks like this
#PV = nRT#, where
#P# - the pressure of the gas;
#V# - its volume;
#n# - the number of moles of gas;
#R# - the ideal gas constant, usually expressed in atm L/mol K;
#T# - the temperature of the gas in Kelvin.
You know that your gas occupies a volume of 2 L at a pressure of 2 atm and a temperature of #20^@"C"#. Plug these values into the ideal gas lw equation and solve for #n#:
#PV = nRT => n = (PV)/(RT)#
#n = (2cancel("atm") * 2cancel("L"))/(0.082(cancel("atm") * cancel("L"))/("mol" * cancel("K")) * (273.15 + 20)cancel("K")) = "0.1664 moles"#
SIDE NOTE Do not forget to convert the temperature to Kelvin! You do that by adding 273.15 to the value given in degrees Celsius, like I did above.
Rounded to one sig fig, the answer will be
#n = color(green)("0.2 moles")#
