How many number of aluminium ions are present in 0.051 g of aluminium oxide?

Molar mass of Al₂O₃  = 2 x 27 + 3 x 16 = 102 g
0.051 g of Al₂O₃  $=\frac{0.051}{102}=0.0005 \mathrm{mol}$
1 mole of Al₂O₃ contains 2 x 6.023 x 10²³ Al³⁺ ions
0.0005 moles of Al₂O₃ contains 2 x 0.0005 x 6.023 x 10²³
Al³⁺ ions = 6.023 x 10²⁰ Al³⁺ ions