How many moles of hydrogen gas occupy a volume of 0.25L at STP?

There are 0,011 moles of hydrogen gas.

Explanation:

We use the ideal gas formula, with the constant R = 0.082 l atm / K mol. The STP conditions are : 1 atm pressure and 273 K temperature. Solve for the formula, n (number of moles):

PV=nRT ---> n= (PV)/(RT)

n= (1 atm x 0,25 L)/ (0,082 l atm/ K mol x 273 K)

n= 0,011 mol