Question

calculate the percent composition by mass of each element in al(oh)₃. use at least three significant figures.
% al =
% o =
% h =

Explanation:

Step1: Calculate molar mass of Al(OH)₃

The molar mass of Al is $M_{Al}=26.98\ g/mol$, O is $M_O = 16.00\ g/mol$ and H is $M_H=1.01\ g/mol$. In $Al(OH)_3$, there is 1 Al, 3 O and 3 H. The molar mass of $Al(OH)_3$ is $M = 26.98+3\times16.00 + 3\times1.01=26.98 + 48.00+3.03 = 78.01\ g/mol$.

Step2: Calculate percent - mass of Al

The percent - mass of Al is $\%Al=\frac{26.98}{78.01}\times100\% \approx 34.6\%$.

Step3: Calculate percent - mass of O

The percent - mass of O is $\%O=\frac{3\times16.00}{78.01}\times100\%=\frac{48.00}{78.01}\times100\% \approx 61.5\%$.

Step4: Calculate percent - mass of H

The percent - mass of H is $\%H=\frac{3\times1.01}{78.01}\times100\%=\frac{3.03}{78.01}\times100\% \approx 3.88\%$.

Answer:

% Al = 34.6%
% O = 61.5%
% H = 3.88%