# Question 67109

May 15, 2015

The percent of aluminium in aluminium sulfate, $A {l}_{2} {\left(S {O}_{4}\right)}_{3}$, is 15.8%.

The percent composition of aluminium in aluminium sulfate refers to the amount of aluminium you get for every 100 g of aluminium sulfate.

You get the amount of aluminium by using moles and molar masses. Notice that 1 mole of $A {l}_{2} {\left(S {O}_{4}\right)}_{3}$ dissociates to produce 2 moles of aluminium cations

$A {l}_{2} {\left(S {O}_{4}\right)}_{3} \to \textcolor{red}{2} A {l}^{3 +} + 3 S {O}_{4}^{2 -}$

This means that you have

(color(red)(2) * 26.9815 cancel("g/mol"))/(342.15cancel("g/mol")) * 100 = color(green)("15.8%")#, where

$\text{26.9815 g/mol}$ - the molar mass of aluminium;
$\text{342.15 g/mol}$ - the molar mass of aluminium sulfate.

This means that you get 15.8 g of aluminium for every 100 g of aluminium sulfate.