Answer: A (\((2.4/16) \times (2/3)\)). The oxide has formula Y₂O₃, so the mole ratio of Y to O is 2 : 3. Start from the oxygen. The mass of oxygen in the 8.0 g of Y₂O₃ is \(8.0 - 5.6 = 2.4\) g, and the relative atomic mass of oxygen is 16, so moles of O \(= 2.4/16\).
Using the formula ratio Y : O \(= 2 : 3\), the moles of Y are \(\tfrac{2}{3}\) of the moles of O. Therefore moles of Y \(= (2.4/16) \times (2/3)\), which is option A.
Working it through, \(2.4/16 = 0.15\) and \(0.15 \times 2/3 = 0.10\) mol of Y, which from 5.6 g implies \(A_r(\text{Y}) = 56\), consistent with iron in Fe₂O₃. The key trap is the ratio: option B inverts it to 3/2, and options C and D wrongly start from the total 8.0 g instead of the oxygen mass.
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