Use the ideal gas law to determine moles of "H"_2"H2. Then multiply the moles times the molar mass. "STP=273.15 K and 100 kPa"STP=273.15 K and 100 kPa.
Part 1: Ideal Gas Law
PV=nRTPV=nRT, where nn represents moles and RR is the gas constant.
Given/Known
P="100 kPa"P=100 kPa
V="56 L"V=56 L
R="8.3144598 L kPa K"^(-1) "mol"^(-1)"R=8.3144598 L kPa K−1mol−1
T="273.15 K"T=273.15 K
Unknown
nn
Solution
Rearrange the equation to isolate nn and solve.
n=(PV)/(RT)n=PVRT
n=(100cancel"kPa"xx56cancel"L")/(8.3144598cancel"L" cancel"kPa" cancel"K"^(-1) "mol"^(-1)xx273.15cancel"K")="2.5 mol H"_2" (rounded to two significant figures)
Part 2: Mass of "H"_2"
Multiply the moles of hydrogen gas times its molar mass, "2.01588 g/mol".
2.5cancel"mol H"_2xx(2.01588"g H"_2)/(1cancel"mol H"_2)="5.0 g H"_2"