Question

What is the mass of 7.0 L of hydrogen gas?

Answer 1

In order to determine the mass of any gas you need the set of paramaters that describe that gas - pressure, volume, temperature and/or moles. If values are given for these paramaters, you could use the ideal gas law, `PV = nRT`, to go about solving for mass.

If you are dealing with STP conditions, which I assume is the case here, remember that in this case pressure is equal to 1.00 atm and temperature is equal to 273.15K. So,

`PV = nRT -> n = (PV)/(RT) = (1.00 atm * 7.0L)/(0.082 (atm * L)/(mol*K) * 273.15K) = 0.31`

Since we know hydrogen's molar mass (`2.0 g/(mol)`), we can determine the mass

`m_(H_2) = n * molar.mass = 0.31 mol es * 2.0g/(mol) = 0.62` `g`

If indeed you are dealing with STP, remember that, under these conditions, 1 mole of any ideal gas occupies `22.4L`. So,

`n = V/V_(molar) = (7.0L)/(22.4L) = 0.31` moles

And, once again, `m = 0.31 * 2.0 = 0.62g`