Question

what is the percent by mass of aluminum in aluminum sulfate, al₂(so₄)₃? (molar mass of al₂(so₄)₃ = 342.14 g/mol)
15.77 %
7.886 %
21.93 %
35.94 %
45.70 %

Explanation:

Step1: Determine Al molar - mass contribution

In $Al_2(SO_4)_3$, there are 2 Al atoms. Molar mass of Al is 26.98 g/mol, so mass of Al in $Al_2(SO_4)_3$ is $2\times26.98 = 53.96$ g/mol.

Step2: Calculate mass - percent formula

The formula for mass - percent is $\text{Mass}\%=\frac{\text{Mass of element}}{\text{Molar mass of compound}}\times100\%$.
Substitute the values: $\text{Mass}\%=\frac{53.96}{342.14}\times 100\%$.
$\frac{53.96}{342.14}\times100\% \approx 15.77\%$

Answer:

15.77%