Answer:
34.59 % Al; 61.54 % O; 3.877 % H
Explanation:
% by mass = mass of element/total mass × 100 %
Let's rewrite the formula of Al(OH)₃ as AlO₃H₃.
Set up a table for easy calculation.
Atom Mass/u Subtotal
1Al 1 × 26.98 = 26.98 u
3O 3 × 16.00 = 48.00 u
3H 3 × 1.008 = 3.024 u
Total = 78.004 u
Mass % Al = 26.98/78.004 × 100 % = 34.59 %
Mass % O = 48.00/78.004 × 100 % = 61.54 %
Mass % H = 3.024/78.004 × 100 % = 3.877 %
