"determine the mass of oxygen" in a 7.9 g sample of al2(so4)3.

              4.43 g of Oxygen

Explanation:

As shown in Chemical Formula, one mole of Aluminium Sulfate [Al₂(SO₄)₃] contains;

                          2 Moles of Aluminium

                          3 Moles of Sulfur

                          12 Moles of Oxygen

Also, the Molar Mass of Aluminium Sulfate is 342.15 g/mol. It means,

          342.15 g ( 1 mole) of Al₂(SO₄)₃ contains  =  192 g (12 mole) of O

So,

                         7.9 g of Al₂(SO₄)₃ will contain  =  X g of O

Solving for X,

                       X  =  (7.9 g × 192 g) ÷ 342.15 g

                      X =  4.43 g of Oxygen

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