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")#