The label has come off a cylinder of gas. To determine the gas in the cylinder, you evacuate $($empty$)$ a $5L$ flask of known weight, then let gas from the cylinder flow into it until the gauge pressure is $1$atm. After weighing the flask with the added gas, you determine the weight of the gas to be $34\mathrm{.}7g.$ Room temperature is $70F$ and the pressure in the room is $1$atm. What is the gas?