Blog: What is the percent of aluminum in Al2(SO4)3?

Hello!

Probably the question is about mass: what percent does aluminium constitute of the mass of a molecule $\displaystyle{A}{l}_{{2}}{\left({S}{O}_{{4}}\right)}_{{3}}?$

To calculate this, we should know the atomic masses of $\displaystyle{A}{l}$ (aluminium), $\displaystyle{S}$ (sulfur) and $\displaystyle{O}$ (oxygen) atoms in some units. We can choose any units but the same for all types of atoms.

The most convenient unit in this situation is the relative atomic mass. It is defined as $\displaystyle\frac{{1}}{{12}}$ of the...

Hello!

Probably the question is about mass: what percent does aluminium constitute of the mass of a molecule $\displaystyle{A}{l}_{{2}}{\left({S}{O}_{{4}}\right)}_{{3}}?$

The most convenient unit in this situation is the relative atomic mass. It is defined as $\displaystyle\frac{{1}}{{12}}$ of the mass of an atom of carbon-12. We may look the periodic table to see that

$\displaystyle{m}{\left({A}{l}\right)}\approx{27},$ $\displaystyle{m}{\left({S}\right)}\approx{32},$ $\displaystyle{m}{\left({O}\right)}\approx{16}.$

Then the mass of a molecule of $\displaystyle{A}{l}_{{2}}{\left({S}{O}_{{4}}\right)}_{{3}}$ is

$\displaystyle{2}\cdot{m}{\left({A}{l}\right)}+{3}\cdot{\left({m}{\left({S}\right)}+{4}\cdot{m}{\left({O}\right)}\right)}\approx{342}.$

Aluminium in such a molecule has the mass $\displaystyle{2}\cdot{m}{\left({A}{l}\right)}\approx{54}.$ Thus the percent in question is $\displaystyle\frac{{54}}{{342}}\cdot{100}%\approx{15},{79}%,$ so the answer is 15.8%.