Question

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

(a) ${\mathrm{CaCO}}_3$

(b) ${\mathrm{MgCl}}_2$

(c) ${\mathrm{H}}_2{\mathrm{SO}}_4$

Answer 1

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

${\mathrm{CaCO}}_3$:

Writing the atomic masses,

$$\begin{gathered} \mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{Ca}=40 \\ \mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{C}=12 \\ \mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{O}=16 \\ \mathrm{Ca}:\mathrm{C}:(\mathrm{O}\times 3) \\ 40:12:(16\times 3) \\ 40:12:48 \end{gathered}$$

${\mathrm{MgCl}}_2$:

Writing the atomic masses,

$$\begin{gathered} \mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{Mg}=24 \\ \mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{Cl}=35.5 \\ \mathrm{Mg}:(\mathrm{Cl}\times 2) \\ 24:(35.5\times 2) \\ 24:71 \end{gathered}$$

${\mathrm{H}}_2{\mathrm{SO}}_4$:

Writing the atomic masses,

$$\begin{gathered} \mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{H}=1 \\ \mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{S}=32 \\ \mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{O}=16 \\ (\mathrm{H}\mathrm{x}2):\mathrm{S}:(\mathrm{O}\times 4) \\ 2:32:(16\times 4) \\ 2:32:64 \\ 1:16:32 \end{gathered}$$

• The ratio by mass of the combining elements in ${\mathrm{CaCO}}_3$ is 40: 12: 48.
• The ratio by mass of the combining elements in ${\mathrm{MgCl}}_2$ is 24: 71.
• The ratio by mass of the combining elements in ${\mathrm{H}}_2{\mathrm{SO}}_4$ is 1: 16: 32.