**Category:** Inorganic Chemistry  
**Subtype:** Chemical Reactions (Combustion Reaction)  
**Problem Description:** Find the mass of aluminum (Al) that was completely burned in excess oxygen (O₂) to produce a given mass of aluminum oxide (Al₂O₃).  

**Given Data:** The mass of Al₂O₃ produced is 20.4 grams.  
**Objective:** Determine the mass (m) of aluminum (Al) used in the reaction.