Chapter 10, Problem 214A

Interpretation Introduction

Interpretation:

The chemical formula and percent composition of luminol needs to be determined.

Concept introduction:

The chemical composition of a substance is expressed by its chemical formula, which uses atomic symbols of the elements and uses subscripts to indicate the number of atoms present in the compound of an element.

The composition of a substance represented in terms of percentage of each element in comparison to the total mass of the compound is said to be percent composition.

Answer to Problem 214A

The chemical formula of luminol is ${\text{C}}_{\text{8}}{\text{H}}_{\text{7}}{\text{N}}_{\text{3}}{\text{O}}_{\text{2}}$ and percent composition of luminol is:

$\text{Percent composition of C = }54.23\%$

$\text{Percent composition of H = }3.99\%$

$\text{Percent composition of N = 23}\text{.72 }\%$

$\text{Percent composition of O = 18}\text{.05 }\%$

Explanation of Solution

The structure of luminol is:

This structure can be rewritten as a complete structure as:

In order to determine the chemical formula, the number of atoms of each element in the compound is calculated as:

$\begin{array}{l}\text{Carbon, C = 8}\\ \text{Hydrogen, H = 7}\\ \text{Oxygen, O = 2}\\ \text{Nitrogen, N = 3}\end{array}$

Thus, the chemical formula of luminol is ${\text{C}}_{\text{8}}{\text{H}}_{\text{7}}{\text{N}}_{\text{3}}{\text{O}}_{\text{2}}$.

The molar mass is sum of the atomic mass of all the elements present in the compound so, the molar mass of luminol, ${\text{C}}_{\text{8}}{\text{H}}_{\text{7}}{\text{N}}_{\text{3}}{\text{O}}_{\text{2}}$ is:

${\text{Molar mass of C}}_{\text{8}}{\text{H}}_{\text{7}}{\text{N}}_{\text{3}}{\text{O}}_{\text{2}}\text{ = Atomic mass of }\left[\text{8(C) + 7(H) + 3(N) + 2(O)}\right]$

Substituting the values:

$\begin{array}{l}{\text{Molar mass of C}}_{\text{8}}{\text{H}}_{\text{7}}{\text{N}}_{\text{3}}{\text{O}}_{\text{2}}\text{ = }\left[\text{8(12}\text{.01) + 7(1}\text{.01) + 3(14}\text{.01) + 2(15}\text{.99)}\right]\text{ g/mol}\\ {\text{Molar mass of C}}_{\text{8}}{\text{H}}_{\text{7}}{\text{N}}_{\text{3}}{\text{O}}_{\text{2}}\text{ = 177}\text{.16 g/mol}\end{array}$

Now, the percent composition is determined using formula:

$\text{Percent composition of element = }\frac{\text{Mass of element in 1 mole of compound}}{\text{Molar mass of compound}}\times 100\%$

Since, in 1 mole of luminol there are 8 moles of C, 7 moles of H, 3 moles of N and 2 moles of O so, the composition of each element will be:

$\begin{array}{l}\text{Percent composition of C = }\frac{\text{8(12}\text{.01)}}{\text{177}\text{.16}}\times 100\%\\ \text{Percent composition of C = }54.23\%\end{array}$

$\begin{array}{l}\text{Percent composition of H = }\frac{\text{7(1}\text{.01)}}{\text{177}\text{.16}}\times 100\%\\ \text{Percent composition of H = }3.99\%\end{array}$

$\begin{array}{l}\text{Percent composition of N = }\frac{\text{3(14}\text{.01)}}{\text{177}\text{.16}}\times 100\%\\ \text{Percent composition of N = 23}\text{.72 }\%\end{array}$

$\begin{array}{l}\text{Percent composition of O = }\frac{\text{2(15}\text{.99)}}{\text{177}\text{.16}}\times 100\%\\ \text{Percent composition of O = 18}\text{.05 }\%\end{array}$