# A 4.78-g sample of aluminum completely reacts with oxygen to form 6.67 g of aluminum oxide. What is the mass percent composition of aluminum in aluminum oxide?

Oct 30, 2015

71.7%

#### Explanation:

The percent composition of aluminium in the

resulting oxide will be the ratio between the mass of aluminijm and the total mass of the oxide, multiplied

by $100$.

$\text{% Al" = m_"Al"/m_"oxide} \times 100$

In your case, you know that a $\text{4.78-g}$ sample of aluminium reacts completely with excess oxygen to form an

unknown aluminium oxide.

Moreover, you know that this oxide has a mass of $\text{6.67 g}$.

Since all the luminium that initially reacted with the oxygen is now a part of the oxide, you can say that its

percent composition in the oxide will be

"% Al" = (4.78color(red)(cancel(color(black)("g"))))/(6.67color(red)(cancel(color(black)("g")))) xx 100 = 71.664%

Rounded to three sig figs, the

"% Al" = color(green)(71.7%)