① Relative atomic mass comparison method: recommended, relatively simple.

Because H2 at standard temperature and pressure is 5.6 liters

So n(H2)= = 5.6/22.4 = = 0.25 mol.

And because the mass of the mixture is 12g.

So the average relative molecular weight of the mixture is 12/0.25==48.

Because 1mol of Na reacts with H2O to release 0.5mol of H2, the relative atomic mass of 2na-H2, that is, 2 mol of Na ==2×23==46, does not exceed 48.

Similarly, 1 mol of magnesium reacts with HCl to release 1 mol of H2.

That is, relative atomic mass 1molMg ==24.

Al is 27/ 1.5== 18.

Fe is 56/ 1==56.

So only Fe is greater than 48.

So it must contain iron.

② Quality comparison method:

Because H2 at standard temperature and pressure is 5.6 liters

So n(H2)= = 5.6/22.4 = = 0.25 mol.

Sodium does not react with hydrochloric acid to release H2, but reacts with water to release H2.

Because 1mol of Na reacts with H2O to release 0.5mol of H2.

1mol Mg reacts with HCl to release 1molH2.

1mol Al reacts with HCl to release 1.5molH2.

1mol Fe reacts with HCl to release 1molH2.

If it contains only one metal,

Na needs 0.5mol, and the mass is11.5g. ..

Mg needs 0.25mol and the mass is 6g.

Al needs 1/6mol. The mass is 4.5g

Fe needs 0.25mol and the mass is 14g.

Because the mixture is 12g, there must be Fe with a mass greater than 12g.