Calculate the number of aluminium ions present in 0.051g 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 = 27u)

Solution:

To calculate the number of aluminium ions in 0.051g of aluminium oxide,

1 mole of aluminium oxide = 6.022 x 10²³ molecules of aluminium oxide

1 mole of aluminium oxide (Al₂O₃) = 2 x Mass of aluminium + 3 x Mass of oxygen

= (2x 27) + (3 x16) = 54 +48 = 102g

1 mole of aluminium oxide = 102g = 6.022 x 10²³ molecules of aluminium oxide

Therefore, 0.051g of aluminium oxide has = 6.022 x 10²³ / 102 x 0.051

= 3.011 x 10²⁰ molecules of aluminium oxide

One molecule of aluminium oxide has 2 aluminium ions; hence, the number of aluminium ions present in 0.051g of aluminium oxide = 2 x 3.011x 10²⁰ molecules of aluminium oxide.

= 6.022 x 10²⁰