Answer:
Mass = 40 g
Step-by-step explanation:
Given data:
Mass of Mg = 24 g
Mass of O₂ = 32 g
Moles of MgO = ?
Chemical equation:
 2Mg + O₂ → 2MgO
Number of moles of Mg:
Number of moles of Mg= Mass /molar mass
Number of moles of Mg= 24g / 24/mol
Number of moles of Mg= 1 mol
Number of moles of O₂ :
Number of moles of O₂ = Mass /molar mass
Number of moles of O₂ = 32g / 32 g/mol
Number of moles of O₂ = 1 mol
Now we will compare the moles of water with hydrogen and oxygen .
 Mg : MgO
 2 : 2
 1 : 1
 O₂ : MgO
 1 : 2
The number of moles of MgO produced by Mg are less it will be limiting reactant.
Mass of MgO:
Mass = number of moles × molar mass
Mass = 1 mol × 40 g/mol
Mass = 40 g