Consider the reaction.

which statement is true at stp? (the atomic mass of mg is 24.31 u.)
3.0 g of mg produce 3.0 l of h2 gas.
3.0 g of mg produce 22.4 l of h2 gas.
24.31 g of mg produce (3 22.4) l of h2 gas.
(3 24.31) g of mg produce (3 22.4) l of h2 gas.

Answer is: (3 24.31) g of Mg produce (3 22.4) L of H2 gas.

Balanced chemical reaction:

3Mg(s) + 2H₃PO₄(aq) → Mg₃(PO₄)₂(s) + 3H₂(g).

From chemical reaction: n(Mg) : n(H₂) = 3 : 3 (1 : 1).

n(Mg) = n(H₂).

m(Mg) = 24.31 g; mass of magnesium.

n(Mg) = m(Mg) ÷ M(Mg).

n(Mg) = 24.31 g ÷ 24.31 g/mol.

n(Mg) = 1 mol; amount of the magnesium.

V(H₂) = n(H₂) · Vm.

V(H₂) = 1 mol · 22.4 L/mol.

V(H₂) = 22.4 L; volume of the hydrogen gas.

So, 3·24.31 g of magnesium (3 mol) produce 3·22.4 liters of hydrogen.

a corroding iron, if that's one of your choices