Question

Find the nitrogen percentage in $\left[ {{\text{CO}}{{\left( {{\text{N}}{{\text{H}}_{\text{2}}}} \right)}_{\text{2}}}} \right]$ .
A.$23$
B.$28$
C.$46.67$
D.$25$

Answer 1

Hint: The percent composition of a molecule by mass refers to the proportion of a particular molecule which consists of the constituent elements of that molecule. The percentage of a constituent element of a molecule can be determined by dividing the mass of that element by the molecular mass of the molecule, followed by its multiplication by 100.

Complete step by step answer:
The given molecule is urea molecule and its chemical formula is $\left[ {{\text{CO}}{{\left( {{\text{N}}{{\text{H}}_{\text{2}}}} \right)}_{\text{2}}}} \right]$ . Thus, the molecule of urea is having only one carbon atom and one oxygen atom but there are two nitrogen atoms in it. Also, there are 4 hydrogen atoms present.
Let us determine the atomic mass of each element present in the molecule of urea.
From the periodic table, it can be seen that the atomic masses of the element hydrogen is 1, that of the element nitrogen is 14, that of carbon is 12 and that of oxygen is 16.
Now, let us determine the mass of each individual element present in the urea molecule.
The mass of an individual element is obtained by the multiplication of the atomic mass with the atomicity or the number of atoms of that element.
Atomicity of carbon is 1 and so the mass of carbon is $1 \times 12 = 12{\text{g/mol}}$ .
Atomicity of oxygen is 1 and so the mass of oxygen is $1 \times 16 = 16{\text{g/mol}}$ .
Atomicity of nitrogen is 2 and so the mass of nitrogen is $2 \times 14 = 28{\text{g/mol}}$ .
Atomicity of hydrogen is 4 and so the mass of hydrogen is $4 \times 1 = 4{\text{g/mol}}$ .
The molecular weight of the urea molecule is then obtained by the sum of the figures obtained above.
$12 + 16 + 28 + 4 = 60{\text{g/mol}}$
Thus, the percentage of nitrogen in urea is
$
   = \dfrac{{28}}{{60}} \times 100 \\
   = 46.67 \\
 $

Hence, the correct option is C.

Note:
Urea is mainly used in agriculture as nitrogen releasing fertilizer. It possesses the highest content of nitrogen of all solid nitrogenous fertilizers. These urea fertilizers rapidly convert into ammonium ions as the urease enzyme present in soil bacteria catalyses the conversion of urea into ammonium ions.
Urea is also used in the chemical industry as a raw material for the preparation of urea-formaldehyde resins and urea-melamine-formaldehyde.