Question

Find the ratio by mass of the combining elements in the following compounds.

(a) ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}$

(b) ${\mathrm{NH}}_3$

(c) $\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$

Answer 1

• The phrase "molecular mass" refers to the entire sum of the masses of the atoms or components that make up the molecule.

${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}$:

Writing the atomic masses,

$$\begin{gathered} \mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{C}=12 \\ \mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{H}=1 \\ \mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{O}=16 \\ (\mathrm{C}\times 2):(\mathrm{H}\times 6):\mathrm{O} \\ (12\times 2):(1\times 6):16 \\ 24:6:16 \\ 12:3:8 \end{gathered}$$

${\mathrm{NH}}_3$:

Writing the atomic masses,

$$\begin{gathered} \mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{N}=14 \\ \mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{H}=1 \\ \mathrm{N}:(\mathrm{H}\times 3) \\ 14:(1\times 3) \\ 14:3 \end{gathered}$$

$\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$:

Writing the atomic masses,

$$\begin{gathered} \mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{Ca}=40 \\ \mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{H}=1 \\ \mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{O}=16 \\ \mathrm{Ca}:\mathrm{O}\times 2:\mathrm{H}\times 2 \\ 40:(16\times 2):(1\times 2) \\ 40:32:2 \\ 20:16:1 \end{gathered}$$

The ratio by mass of the combining elements in ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}$ is 12: 3: 8.

The ratio by mass of the combining elements in ${\mathrm{NH}}_3$ is 14:3.

The ratio by mass of the combining elements in $\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$ is 20: 16: 1.