Calculate the percent composition by mass of each element in al(oh)3. use at least three significant figures.

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 %

the answer is a! (2.9 x 10^-18 j) hope i could someone. : )