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")
