Question

What is the percent composition by mass of aluminium in a compound of aluminium and oxygen if the mole ratio of $Al:O$ is $2:3$?

• A. $37$% $Al$
• B. $47$% $Al$
• C. $53$% $Al$
• D. $63$% $Al$
• E. $74$% $Al$

Answer 1

The correct option is C $53$% $Al$

In a compound of aluminium and oxygen the mole ratio of $Al:O$ is $2:3$
The atomic masses of $Al$ and $O$ are 27.0 g/mol and 16 g/mol respectively.
2 moles of Al $=2\times 27.0=54.0$ g
3 moles of O $=3\times 48.0$
Hence, the percent composition by mass of aluminium is $\frac{54.0}{54.0+48.0}\times 100=53$%.