How many moles are in 5 L of #H_2# at STP?
For this type of problem you would use the ideal gas law equation, PV = nRT. Where P represents pressure (must have units of atm), V represents volume (must have units of liters), n represents the number of moles, R is the proportionality constant
(has units of L atm/ mol K), and T represents the temperature, which must be in Kelvins.
Now what you want to do is list your known and unknown variables. Our only unknown is the number of moles of hydrogen. Our known variables are P,V,R, and T. Since we are at STP, the temperature is 273K and the pressure is 1 atm. We are given volume and the proportionality constant, R, is equal to 0.0821 L atm/ mol K.
Now all we have to do is rearrange the equation and solve for n like so:
n = PV/RT
n = (1 atm) (5 L) / (0.0821 L atm/ mol K) (273K)
Which gives you 0.223 moles of hydrogen gas.
I hope this makes sense, and here is a link for additional help!