Given: At. Mass of AL = 27 u

Molar mass of aluminium oxide (Al₂O₃) = 2 × 27 + 3× 16 = 54 + 48 = 102g

This means 102g of Al₂O₃ has 2 × 6.022 × 10²³ molecules

Then 0.051g contains:

⇒ Number of Al³⁺ ions = 6.022 × 10²⁰

Thus, 0.051 g of aluminium oxide contains 6.022 × 10²⁰ aluminium ions.