Question

calculate no. of Al atoms present in 0.5 mole of aluminium oxide (Al2O3)

Answer 1

Do you mean Al2O3?

Let us compute the molar mass of the molecule first

Mass of Al = 27 AMU

Mass of O = 16 AMU

So mass of Al2O3 "molecule" = 2x27 + 3x16 = 102 AMU

So 1 mole of  ALUMINIUM OXIDE will weigh 102 g

1 mole of any pure substance contains Avogrado's number of molecules, that is 6.023x10^23.

Thus 0.051g of ALUMINIUM OXIDE will contain 6.023x(10^23)x.051/102 = 6.023x(10^20)/2 "molecules "

Each molecule contains two ions of aluminium.

So total no. Of ions in the given amount of substance =  2x6.023x(10^20)/2 = 6.023x10^20