Chapter 3: Atoms and Molecules

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Atoms and Molecules


Q.11: 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)


The chemical formula of aluminium oxide is Al2O3. The given mass of aluminium oxide is 0.051g.

Molar mass of Al2O3 = 2(Atomic mass of Al) + 3(Atomic mass of O)

= 2(27g) + 3(16g) = 102g

So, the mass of 1 mole of Al2O3 is 102g. One mole of elements contains 6.023x1023 atoms. This means that the mass of 6.023x1023 atoms is 102g.

So, the number of molecules in 0.051 g of Al2O3

mathbf{=frac { 6.023	imes { 10 }^{ 23 } }{ 102g } 	imes 0.051g=3.012	imes { 10 }^{ 20 } molecules}


Thus, 0.051 g of Al2O3 contains 3.012x1020 molecules. One molecule of Al2O3 contains 2 Al3+.

So, the number of Al3+ ions in 3.012x1020 molecules of Al2O3

=2	imes 3.012	imes 10^{20} =6.023	imes 10^{20} Al3+ ions.

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