The % oxygen by mass of a Group 2 oxide is 39.7%. Name the Group 2 element.

Question

BlackTom AI · Accepted Answer

The question asks to name the Group 2 element given that its oxide has 39.7% oxygen by mass.
First, recall the relevant oxides for Group 2 elements: BeO, MgO, CaO, SrO, BaO, etc., and their formula masses.
To check, compute the oxygen mass percentage for magnesium oxide: MgO has formula mass approximately 24.305 (Mg) + 16.000 (O) = 40.305 g/mol. The oxygen fraction is 16.000 / 40.305 × 100% ≈ 39.7%.
Now compare with other Group 2 oxides to see if the percentage would match 39.7%:
- BeO: Be is about 9.012 g/mol, so formula mass ≈ 9.012 + 16.000 = 25.012 g/mol; O percentage ≈ 16.000 / 25.012 × 100% ≈ 64%.
- CaO: Ca is about 40.08 g/mol, so formula mass ≈ 40.08 + 16.000 = 56.08 g/mol; O percentage ≈ 16.000 / 56.08 × 100% ≈ 28.6%.
These percentages differ significantly from 39.7%, so neither Be nor Ca oxides match the given data.
Since the computed O mass fraction for MgO matches the given 39.7%, the element in question must be magnesium (Mg).
Note: The problem statement provides the correct answer as Mg, and this aligns with the calculation above. If the options were broader, you would apply the same mass-percentage approach to each oxide to identify the one whose oxygen mass fraction equals 39.7%.

The % oxygen by mass of a Group 2 oxide is 39.7%. Name the Group 2 element.

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