Question

How do you calculate the percent composition by mass of each element in `Al(OH)_3`?

Answer 1

Add up all of the elemental atomic weights for the compound mass. Then divide each of the individual element weights in the compound by that mass to find their mass percentage.

Explanation:

Be sure to include ALL of the elements indicated by the subscripts!
Al = 27
(OH)3 means THREE (3) EACH of O and H !
O = 16 16 * 3 = 48
H = 1 1 * 3 = 3
Al(OH)3 = 78
%O = 48/78
%H = 3/78
%Al = 27/78