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Kevlar, used in making bulletproof vests, has the percentage composition: C = 70.6%, H = 4.2%, N = 11.8%, O = 13.4%. Determine its empirical formula.

1. C7H5NO2
2. C7H5N2O
3. C7H9NO
4. C7H5NO

Correct answer: C7H5NO

Solution

Moles of each element per 100 g: C = 70.6/12 = 5.883, H = 4.2/1 = 4.2, N = 11.8/14 = 0.843, O = 13.4/16 = 0.8375. Divide by smallest (0.8375): C = 5.883/0.8375 ≈ 7.02 ≈ 7; H = 4.2/0.8375 ≈ 5.01 ≈ 5; N = 0.843/0.8375 ≈ 1.007 ≈ 1; O = 0.8375/0.8375 = 1. Empirical formula: C7H5NO.

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