Question

When magnesium is heated in air, it reacts with oxygen to form magnesium oxide. Suppose a piece of magnesium reacts with oxygen and the mass of the solid increases by 2.00 g. How many grams of magnesium reacted?

Answer 1

Reaction:

2 Mg (s) + O₂(g)   ------> 2 MgO(s)

The mass of solide increased is equal to the mass of oxygen reacted.

So, the mass of oxygen reacted is 2.00 g.

Moles of O₂ = mass of O₂/molar mass of O₂ = 2.00 g/(32 g/mol) = 0.0625 mol

From balanced equation, mole ratio of Mg and O₂ is 2:1.

Moles of Mg reacted = 2 x( moles of O₂ reacted) = 2 x( 0.0625 mol) = 0.125 mol

Mass of Mg reacted = moles of Mg x atomic mass of Mg = 0.125 mol x (24.31 g/mol) = 3.04 g.