Question

What is the percent composition by mass of aluminum in Al2(SO4)3 (gram-formula mass = 342 grams/mole)?
(1) 7.89% (3) 20.8%
(2) 15.8% (4) 36.0%

Answer 1

The atomic mass of Aluminum is around 27 amu. There are two aluminum atoms, so it takes 54 amu. The gram-formula states that 1 mole of this compound has 342 grams. By definition, this means that the total amu of this compound is 342. Therefore, we divide 54 by 342 to get around 15.8 %. 

Answer 2

Answer:

The correct answer is option (2).

Explanation:

Molar mass of aluminum sulfate = 342 g/mol

Atomic mass of aluminium = 26.98 g/mol

In a molecule of aluminum sulfate there are 2 atoms of aluminium.

Mass of aluminum in aluminium sulfate:

$2\times 26.98 g/mol=53.96 g/mol$

Percent composition by mass of aluminum in $Al_2(SO_4)_3$

$\%=\frac{53.96 g/mol}{342 g/mol}\times 100=15.77\%\approx 15.8\%$

Hence, the correct answer is option (2).