# What is the mass of 7.0 L of hydrogen gas?

Dec 23, 2014

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, $P V = n R T$, 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,

$P V = n R T \to n = \frac{P V}{R T} = \frac{1.00 a t m \cdot 7.0 L}{0.082 \frac{a t m \cdot L}{m o l \cdot K} \cdot 273.15 K} = 0.31$

Since we know hydrogen's molar mass ($2.0 \frac{g}{m o l}$), we can determine the mass

${m}_{{H}_{2}} = n \cdot m o l a r . m a s s = 0.31 m o l e s \cdot 2.0 \frac{g}{m o l} = 0.62$ $g$

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

$n = \frac{V}{V} _ \left(m o l a r\right) = \frac{7.0 L}{22.4 L} = 0.31$ moles

And, once again, $m = 0.31 \cdot 2.0 = 0.62 g$