Question

The molecular mass of an organic compound is 78 and its percentage composition is 92.4%C and 7.6%H . Determine the molecular formula of the compound.

Answer 1

Hint: To calculate the molecular formulae of the compound evaluate the Empirical formula for the compound and calculate the n (number of atoms) for the carbon and hydrogen.

Step by step answer: The empirical formula of a compound gives the ration between the numbers of atoms of different elements present in the molecule of the compound. Empirical formula is calculated by calculating the mass of each element of the compound to the moles using the molar mass of the compound.
Weight of carbon is $ = 92.4\% $ as given in the question
\[\dfrac{{92.4}}{{100}}X78 = 72g\]

Therefore the carbon atom is $72g$

And hydrogen atom is $78g - 72g = 6g$
No of moles of carbon atom $ = $ given weight of carbon atom/atomic weight of carbon atom
No of moles for carbon atom \[ = \dfrac{{72}}{{12}} = 6\]

Similarly,
No of moles for hydrogen atom $ = $ given weight of hydrogen atom/atomic weight of hydrogen atom
No of moles for hydrogen atom $ = \dfrac{6}{1} = 6$
As from the question only two atoms Carbon and hydrogen are present in the composition. The total no of moles for each one are 6.
Hence the molecular formula for given composition is ${C_6}{H_6}$

Therefore, the molecular formula for the composition is ${C_6}{H_6}$

Note: The three different types of chemical formula for the compound are empirical, molecular and structural formula. Empirical formula shows the simplest whole-numbers ratio of atoms in a compound. Molecular formula denotes the number of each type of atom in a molecule. Molecular formula is more significant as it tells total number f each atom type present in a compound whereas empirical formula just reveal the ratio of the different type of atom present in the compound. Lastly, the structural formula shows how the atoms in a molecule are bonded to each other.