Considering the rule of three and molar mass of O, the mass of O²⁻ ions in 3 mol of alumina Al₂O₃ is 96 grams.
By multiplying the number of moles by the subscripts next to each atom, you get the amount of each element present in the compound. IIf the same atom appears more than once in the compound, you must add its amounts.
So, in 1 mole of Al₂O₃, you have 2 ions of O²⁻.
Then you can apply the following rule of three: if 1 mole of the compound contains 2 ions of O²⁻, 3 moles of the compound contains how many ions of O²⁻?
[tex]amount of ions of O^{2-} =\frac{3 moles of Al_{2} O_{3} x2 ions of O^{2-}}{1mole of Al_{2} O_{3} }[/tex]
amount of ions of O²⁻= 6 ions
Being the molar mass of O 16 g/mole, this is the amount of mass that a substance contains in one mole, then the mass of O²⁻ can be calculated as:
6 x 16 g/mole= 96 grams
Finally, the mass of O²⁻ ions in 3 mol of alumina Al₂O₃ is 96 grams.
Learn more about molar mass:
