The mineral benitoite is a gemstone for making jewelry. It has a molecular weight of 413.44 grams/mole and a percent composition of 33.21% Ba, 11.58% Ti, 20.38% Si, and 34.83% O. What is the molecular formula?
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Given:
Molecular weight = 413.44 gm
% of Ba = 33.21 %
% of Ti = 11.58 %
% of Si = 20.38 %
% of O = 34.83 %
To Find:
Molecular formula
Calculation:
- Each 100 gm of compound contains 33.21 gm Ba, 11.58 gm Ti, 20.38 gm Si and 34.83 gm O.
- No of moles of:
Ba = 33.21/137 = 0.242
Ti = 11.58/48 = 0.241
Si = 20.38/28 = 0.728
O = 34.83/16 = 2.177
- Dividing by smallest no, we get:
Ba : Ti : Si : O = 1 : 1 : 3 : 9
- So the empirical formula becomes BaTiSi₃O₉
- Empirical formula mass = 137 + 48 + (28×3) + (16×9) = 413 g
- n = Molecular weight / empirical formula mass = 413.44/413 ≈ 1
⇒ Molecular formula = empirical formula × n = BaTiSi₃O₉
- Hence, the molecular formula is BaTiSi₃O₉.
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