A volume of 18.0 L contains a mixture of 0.250 mole~ N_2 , 0.250 mole O_2 , and an unknown quantity of rm He. The temperature of the mixture is 0…

A volume of 18.0 L contains a mixture of 0.250 mole~ N_2 , 0.250 mole O_2 , and an unknown quantity of rm He. The temperature of the mixture is 0 degrees C , and the total pressure is 1.00 atm . How many grams of helium are present in the gas mixture?