Al2O3 occurs in nature as a mineral called corundum, which is noted for its hardness and resistance to attack by acids. Its density is 3.970 g cm-3. Calculate the number of atoms of Al in 25.00 cm3 of this mineral.

1.17×10²⁴ atoms of Al

Explanation:

Compound: Aluminum oxide (Al₂O₃)

Molar mass = 101.96 g/mol

Density of compound = 3.970 g/cm³ = Mass of compound / Volume of compound

Density . volume = mass → 3.970 g/cm³ . 25 cm³ = 99.25 g

Let's make this relation

1 mol of Al₂O₃ has 2 moles of Al and 3 moles of O it is the same to say that:

101.96 g of Al₂O₃ has 2 moles of Al and 3 moles of O

Our mass of Al₂O₃ is 99.25 g so let's make a rule of three:

 101.96 g of Al₂O₃ have 2 moles of Al

Then, 99.25 g of Al₂O₃ must have (99.25 g . 2) /101.96g = 1.95 moles of Al

Let's count the atoms:

1.95 moles of Al . 6.02×10²³ atoms / 1 mol = 1.17×10²⁴ atoms of Al

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