# How can I find the percent compositions of Al2(SO4)3?

May 29, 2014

To find the percent composition of Al₂(SO₄)₃, you divide the total mass of each atom by the molecular mass and multiply by 100 %.

#### Explanation:

% by mass = $\text{mass of component"/"total mass}$ × 100 %

Mass of 2 Al atoms = 2 Al atoms × $\left(26.98 \text{u")/(1"Al atom}\right)$ = 53.96 u.

Mass of 3 S atoms = 3 S atoms × $\left(32.06 \text{u")/(1"S atom}\right)$ = 96.18 u

Mass of 12 O atoms = 12 O atoms × $\left(16.00 \text{u")/(1"O atom}\right)$ = 192.0 u

Mass of 1 Al₂(SO₄)₂ formula unit = (53.96 + 96.18 + 192.0) u = 342.1 u

% of Al = $\text{mass of Al"/"total mass}$ × 100 % = $\left(53.96 \text{u")/(342.1"u}\right)$ × 100 % =15.77 %

% of S = $\text{mass of S"/"total mass}$ × 100 % = $\left(96.18 \text{u")/(342.14"u}\right)$ × 100 % =28.11 %

% of O = $\text{mass of O"/"total mass}$ × 100 % = $\left(192.0 \text{u")/(342.14"u}\right)$ × 100 % =56.12 %