The molar volume of a gas at STP is #"22.414L/mol"#. You must convert the given volume of hydrogen gas to liters. Then, using the molar volume, convert the liters to moles. Then convert the moles to mass using the molar mass of hydrogen gas, which is #"2.016g/mol"#
Convert mL #"H"_2# to liters.
#"35.0mL H"_2"# x #"1 L"/"1000mL"# = #"0.0350L H"_2#
Convert liters #"H"_2# to moles.
#"0.0350L H"_2# x #"1 mol H"_2/"22.414L H"_2# = #"0.0015615mol H"_2#
Convert moles #"H"_2# tp grams.
#"0.0015615mol H"_2# x #"2.016g H"_2/"1 mol H"_2# = #"0.00315g H"_2#
(The final answer has three significant figures due to three significant figures in #"35.0mL"#.)
At STP, #"35.0mL"# of hydrogen gas has a mass of #"0.00315g"#.
