Subjective Type

Calculating the molecular formula of caffeine? Caffeine contains $$49.48\%$$ Carbon, $$5.15\%$$ Hydrogen, $$28.87\%$$ nitrogen,$$16.49\%$$ oxygen. It has a molar mass of $$194.2$$ gmol -1.

Solution

Percentage of elements in caffeine :
$$C : 49.4\% \ H: 5.15 \%, \ N: 28.87 \%, \ O: 16.49\% $$

To obtain the empirical formula divide each percentage composition by their atomic masses
$$C: 4.12, \ H: 5.15, \ N: 2.06, \ O:1.03$$

Then divide the mole composition with the smallest mole composition i.e., 1.03

Thus the final empirical composition is $$C:4, \ H:5 \ N: 2, \ O:1$$

Empirical formula : $$C_4H_5N_2O$$

Empirical molar mass of the compound obtain : $$ 12 \times 4+ 5 + 2 \times 14 + 16 = 97$$
Given the molar mass of Caffeine = 194.2

$$n= \dfrac{194.2}{97} = 2$$

Thus Formula of Caffeine = $$(C_4H_5N_2O)_2$$

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