Question

Metallic magnesium reacts with steam to produce magnesium hydroxide and hydrogen gas. If 16.2 g of `Mg` are heated with 12.0 g of `H_2O`, what is the limiting reactant?

Answer 1

Water.

Explanation:

Balanced equation of the reaction is
`Mg+2H_2O->Mg(OH)_2+H_2 uarr`

Molar masses of elements in reactants
Atomic mass of Magnesium: 24.3050
Atomic mass of Hydrogen: 1.00794
Atomic mass of Oxygen: 15.9994

Given
12 gm of Water `=12.0/(2xx1.00794+15.9994)approx` 0.6661 moles
And 16.2 g of Magnesium `=16.2/24.3050approx` 0.6665 moles

We observe from the equation that
`1` mol of magnesium requires `2` mols of water.

We require `2xx0.6665=1.333` moles of water to completely consume given quantity of magnesium. We have only 0.6661 moles.
Therefore, water is the limiting reactant.