What is meant by chemical formula? Calculate the number of aluminium ions present in 0.051 g of aluminium oxide.
(Hint: The mass of an ion is the same as that of an atom of the same element. Atomic mass of Al = 27 u)
[5 MARKS]

Each point: 1 Mark

1. The chemical formula of a compound is a symbolic representation of its composition.

2. Mole of aluminium oxide (Al₂O₃) = 2 x 27 + 3 x 16

= 102 g

3. i.e., 102 g of Al₂O₃= 6.022 x 10²³ molecules of Al₂O₃

4. Then, 0.051 g of Al₂O₃ contains = $6.022\times \frac{{10}^{23}}{102}\times 0.051$ molecules

= 3.011 x 10²⁰ molecules of Al₂O₃

5. The number of aluminium ions ((Al³⁺) present in one molecule of aluminium oxide is 2.

Therefore, the number of aluminium ions (Al³⁺) present in 3.011 x 10²⁰ molecules (0.051 g ) of aluminium oxide (Al₂O₃) = 2 x 3.011 x 10²⁰

= 6.022 x 10²⁰