a piece of iron reacts with sulfuric acid and produces 250 dm^3 hydrogen gas at 27 degree celsius temperature and 99.99 kPa. Find out the mass of that piece of iron?

Fe + 2HCl ==> FeCl2 + H2

Use PV = nRT to solve for n = mols gas.
Convert mols H2 to mols Fe (it's 1:1 so mols H2 = mols Fe).
Convert mols Fe to g. g = mols x atomic mass.