Which coefficient in front of O2 yields a balanced equation for the formation of aluminum oxide from aluminum?

• A. 3
• B. 2
• C. 1
• D. 4

Answer: (A)

Balancing a chemical equation means making sure the same number of each type of atom appears on both sides. For aluminum reacting with oxygen to form aluminum oxide, the product is Al2O3, so 2 aluminum atoms and 3 oxygen atoms appear per product unit. To match the aluminum atoms, put a 4 in front of aluminum, giving 4 Al on the left and producing 2 units of Al2O3 on the right (which uses 4 Al). Those 2 Al2O3 units have 6 oxygen atoms in total, so you need 6 oxygen atoms on the left, which come from 3 O2 molecules. Thus the coefficient in front of O2 is 3, yielding the balanced equation: 4 Al + 3 O2 → 2 Al2O3.