14.2 grams
Let's start by looking up the atomic weights of aluminum and oxygen.
Atomic weight aluminum = 26.981539
Atomic weight oxygen = 15.999
Moles Al = 7.5 g / 26.981539 g/mol = 0.277967836
The formula for aluminum oxide is Al2O3, so for every 2 moles of Al, 3 moles of O is required. So let's calculate the number of moles of O we need and from that the mass.
0.277967836 mol / 2 * 3 = 0.416951754 mol
Mass O2 = 0.416951754 * 15.999 = 6.670811105
The mass of aluminum oxide is simply the mass of aluminum plus the mass of oxygen. So:
7.5 g + 6.670811105 g = 14.17081111 g
Rounding to 1 decimal place gives 14.2 g.
