Use the equation for the ideal gas law to find the moles of hydrogen gas. Then determine the mass by multiplying the moles by its molar mass.

**Ideal gas law**

#PV=nRT#

**Known**

#P="80.1 kPa"#

#V="5.00 L"#

#R="8.31447 L kPa K"^(-1) "mol"^(-1)#

#T="20.0"^@"C + 273.15"="293.2 K"#

**Unknown**

moles, #n#

To determine moles #"H"_2"#, rearrange the equation to isolate #n#. Plug in the known values and solve.

#n=(PV)/(RT)#

#n=(80.1color(red)cancel(color(black)("kPa"))xx5.00color(red)cancel(color(black)("L")))/(8.31447color(red)cancel(color(black)("L")) color(red)cancel(color(black)("kPa")) color(red)cancel(color(black)("K"))^(-1) "mol"^(-1)xx293.2color(red)cancel(color(black)("K")))="0.164 mol H"_2"#

To determine the mass of #"H"_2"# by multiplying the moles by its molar mass #("2.016 g/mol")#.

#0.164color(red)cancel(color(black)("mol H"_2))xx(2.016"g H"_2)/(1color(red)cancel(color(black)("mol H"_2)))="0.331 g H"_2"#