**Step 1.** Use the Ideal Gas Law to calculate the moles of hydrogen.

The Ideal Gas Law is

#color(blue)(|bar(ul(PV = nRT)|)#,

where

- #P# is the pressure
- #V# is the volume
- #n# is the number of moles
- #R# is the gas constant
- #T# is the temperature

We can rearrange the Ideal Gas Law to get

#n = (PV)/(RT)#

#P = 750 color(red)(cancel(color(black)("mmHg"))) × "1 atm"/(760 color(red)(cancel(color(black)("mmHg")))) = "0.9868 atm"#

#V = "31.0L"#

#R = "0.082 06 L·atm·K"^"-1""mol"^"-1"#

#T = "(23 + 273.15) K" = "296.15 K"#

#n = (PV)/(RT) = (0.9868 color(red)(cancel(color(black)("atm"))) × 31.0 color(red)(cancel(color(black)("L"))))/( "0.082 06" color(red)(cancel(color(black)("L·atm·K"^"-1"))) "mol"^"-1" × 296.15 color(red)(cancel(color(black)("K")))) = "1.259 mol"#

**Step 2.** Calculate the moles of #"Fe"#.

The balanced equation is:

#"Fe" + "2HCl" → "FeCl"_2 + "H"_2"#

#"Moles of Fe" = 1.259 cancel("mol H"_2) × ("1 mol Fe")/(1 cancel("mol H"_2)) = "1.259 mol Fe"#

**Step 3.** Calculate the mass of #"Fe"#

#"Mass of Fe" = 1.259 color(red)(cancel(color(black)("mol Fe"))) × ("55.84 g Fe")/(1 color(red)(cancel(color(black)("mol Fe")))) = "70.3 g Fe"#