Question

Magnesium combines with oxygen to form magnesium oxide. If 21.56 g of magnesium reacts completely with 5.41 g of oxygen, what is the percent by mass of oxygen in magnesium oxide?

Answer 1

Magnesium combines with oxygen to form magnesium oxide. If $21.56\,\mathrm{g}$ of magnesium reacts completely with $5.41\,\mathrm{g}$ of oxygen, what is the percent by mass of oxygen in magnesium oxide?

$$2\,\mathrm{Mg} + \mathrm{O}_2 \rightarrow 2\,\mathrm{MgO}$$ $$W = \frac{M(O)}{M(Mg) + M(O)} * 100\%$$ $$W = \frac{16}{24 + 16} * 100\% = 40\%$$ $$\mathrm{W} = 40\%$$