Here's how to calculate the percent composition of the aluminum oxide:

1. Find the mass of oxygen:

* Mass of oxygen = Mass of aluminum oxide - Mass of aluminum

* Mass of oxygen = 5.10 g - 2.70 g = 2.40 g

2. Calculate the percentage of aluminum:

* % Aluminum = (Mass of aluminum / Mass of aluminum oxide) * 100%

* % Aluminum = (2.70 g / 5.10 g) * 100% = 52.9%

3. Calculate the percentage of oxygen:

* % Oxygen = (Mass of oxygen / Mass of aluminum oxide) * 100%

* % Oxygen = (2.40 g / 5.10 g) * 100% = 47.1%

Therefore, the percent composition of the aluminum oxide is:

* Aluminum: 52.9%

* Oxygen: 47.1%