Caffeine is made up of 49.5% C, 5.2% H, 16.5% O, and 28.9% N. A solution made up

Question

Caffeine is made up of 49.5% C, 5.2% H, 16.5% O, and 28.9% N. A solution made up of 8.25 g of caffeine and 100.0 mL of benzene (d=0.877 g/mL) freezes at 3.03 C. Pure benzene (Kf=5.10 C/m) freezes at 5.50 C. What are the simplest and molecular formulas for caffeine?

Explanation / Answer

we consider 100 g of this compound Moles C = 49.4 g/ 12.011 = 4.12 Moles H = 5.2 / 1.008 = 5.16 Moles O = 16.5 / 15.9994 = 1.03 Moles N = 28.9 / 14.0067 = 2.06 we divide by the smallest number 4.12 / 1.03 = 4 => C 5.16 / 1.03 = 5 => H 1.03 / 1.03 = 1 => O 2.06 / 1.03 = 2 => N the empirical formula is C4H5ON2 ( mass = 97.04 g/mol) we divide the molar mass by 97.04 and we get 194.1 / 97.04 = 2 we multiply by 2 the empirical formula => C8H10O2N4 is the molecular formula

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