Question

Question: What is the mass of the molecular ion formed from compounds having each molecular formula:

(a) ${\mathbf{C}}_3{\mathbf{H}}_6\mathbf{O}$; (b)${\mathbf{C}}_{10}{\mathbf{H}}_{20}$;(c)${\mathbf{C}}_8{\mathbf{H}}_8{\mathbf{O}}_2$; (d) methamphetamine $\left({\mathbf{C}}_{10}{\mathbf{H}}_{15}\mathbf{N}\right)$?

Short answer

Answer

a. 58

b. 140

c. 136

d. 149

Step-by-step solution

Molecular ion

A molecular ion is formed by the bombardment of the given molecule with electron beams. It is a cation with a single unit positive charge and a radical with an unpaired electron.

Mass of molecular ions

Since the mass of the departing electron is almost negligible concerning the mass of the molecule, the mass of the molecular ion thus formed remains the same as the original molecule.

Determination of the mass

a. The mass of is the total mass of its substituents.

$\begin{array}{c}{\text{C}}_{\text{3}}{\text{H}}_{\text{6}}\text{O\hspace{0.17em}}=3\times 12+6\times 1+1\times 16\\ =36+6+16\\ =58\end{array}$

Hence, the mass of the molecular ion of ${\mathrm{C}}_3{\mathrm{H}}_6\mathrm{O}$ is 58.

b. The mass of is the total mass of its substituents.

$\begin{array}{c}{\text{C}}_{\text{10}}{\text{H}}_{\text{20}}=10\times 12+20\times 1\\ =120+20\\ =140\end{array}$

Hence, the mass of the molecular ion of ${\mathrm{C}}_{10}{\mathrm{H}}_{20}$is 140.

c. The mass of is the total mass of its substituents.

$\begin{array}{c}{\text{C}}_{\text{8}}{\text{H}}_{\text{8}}{\text{O}}_{\text{2}}=8\times 12+8\times 1+2\times 16\\ =96+8+32\\ =136\end{array}$

Hence, the mass of the molecular ion of ${\mathrm{C}}_8{\mathrm{H}}_8{\mathrm{O}}_2$ is 136.

d. The mass of is the total mass of its substituents.

$\begin{array}{c}{\text{C}}_{\text{10}}{\text{H}}_{\text{15}}\text{N\hspace{0.17em}}=10\times 12+15\times 1+1\times 14\\ =120+15+14\\ =149\end{array}$

Hence, the mass of the molecular ion of ${\mathrm{C}}_{10}{\mathrm{H}}_{15}\mathrm{N}$ is 149.