Question

Caffeine has the following percent composition: carbon 49.48%, hydrogen

5.19%, oxygen 16.48% and nitrogen 28.85%. Its molecular weight is 194.19 g/mol.

What is its molecular formula?​

Answer 1

Given:

The percentage composition C=49.48%, N=28.85%, 0=16.48%, H= 5.19%.

To Find:

The empirical formula and a molecular formula of the compound.

Solution:

To find the empirical formula we will follow the following steps:

As we know,

The molecular weight of carbon, oxygen, nitrogen and hydrogen is 12,16, 14 and 1 respectively.

Now,

$Number of moles \: of carbon = \frac{49.48}{12} = 4.12, Number of moles \: of oxygen = \frac{16.48}{16} = 1.03, Number of moles \: of nitrogen = \frac{28.85}{14} = 2.0, Number of moles \: of hydrogen = \frac{5.19}{1} = 5.19$

Diving the number of moles with the smallest containing moles we get carbons, oxygen and hydrogen in the empirical formula.

$Number of carbon = \frac{4.12}{1.03} = 4, Number of oxygen= \frac{1.03}{1.03}= 1, Number of nitrogen= \frac{2.0}{1.03} = 2, Number of hydrogen= \frac{5.19}{1.03} =5$

The empirical formula of the compound is C4H501N2.

Mass of C4H501N2= 97.0

Molecular formula =

$\frac{molecular \: mass}{empirical \: mass}$

$= \frac{194.19}{97.0}$

= 2.

So, the molecular formula is C8H10O2N4.

Henceforth, the molecular formula is C8H1002N4.