# 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?

Feb 11, 2016

Water.

#### Explanation:

Balanced equation of the reaction is
$M g + 2 {H}_{2} O \to M g {\left(O H\right)}_{2} + {H}_{2} \uparrow$

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 $= \frac{12.0}{2 \times 1.00794 + 15.9994} \approx$ 0.6661 moles
And 16.2 g of Magnesium $= \frac{16.2}{24.3050} \approx$ 0.6665 moles

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

We require $2 \times 0.6665 = 1.333$ moles of water to completely consume given quantity of magnesium. We have only 0.6661 moles.
Therefore, water is the limiting reactant.