Question

Question #67109

Answer 1

The percent of aluminium in aluminium sulfate, `Al_2(SO_4)_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 `Al_2(SO_4)_3` dissociates to produce 2 moles of aluminium cations

`Al_2(SO_4)_3 -> color(red)(2)Al^(3+) + 3SO_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

`"26.9815 g/mol"` - the molar mass of aluminium;
`"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.