( P ) ( V ) = ( n ) ( R ) ( T ) n = ( P ) ( V ) / ( R ) ( T ) P = P gauge + P baro = 31.2 psig + 14.8 psia = 46 psia P = ( 46 psia ) ( 1 atm / 14.696 psia ) = 3.13 atm n = ( P ) ( V ) / ( R ) ( T ) n = ( 3.13 atm ) ( 4.6 L ) / ( 0.08206 atm - L / mol - K ) ( 26.0 + 273.2 K ) n = 0.5864 moles m = ( n ) ( M ) m = ( 0.5864 mol ) ( 28.96 g/ mol ) = 16.98 g