A 20g mixture of zinc and magnesium is reacted with a NaOH solution And 5.6 L of hydrogen can be obtained at STP?

Zinc reacts with NaOH, while magnesium does not.
Molar mass of Zn = 65.4 g/mol

Zn(s) + 2NaOH(aq) → Na₂ZnO₂(aq) + H₂(g)
OR: Zn(s) + 2NaOH(aq) + 2H₂O(l) → Na₂Zn(OH)₄(aq) + H₂(g)
In both equations, mole ratio Zn : H₂ = 1 : 1

Molar volume of a gas at STP = 22.4 L/mol
No. of moles of H₂ formed = (5.6 L) / (22.4 L) = 0.25 mol
No. of moles of Zn in the mixture = 0.25 mol

Molar mass of Zn = 65.4 g/mol
Mass of Zn in the mixture = (0.25 mol) × (65.4 g/mol) = 16.35 g

Mass of Mg in the mixture = (20 - 16.35) g = 3.65 g
Mass percentage of Mg in the mixture = (3.65/20) × 100(%) = 18.3%