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%

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 %. 

All the elements are arranged by melting points on the periodic table. in row they go from metals to gasses. in the row from sodium to argon, sodium is more reactive than the one on its right, magnesium.