Question

Calculate the molecular mass of:

Washing soda ${\mathrm{Na}}_2{\mathrm{CO}}_3.10{\mathrm{H}}_2\mathrm{O}$

Answer 1

Molecular mass:

• Molecular mass of a substance is the number of times a molecule of the substance is heavier than $\frac{1}{12}th$ of mass of a carbon ($\mathrm{C}-12$) atom.
• Molecular mass of a substance can be obtained by adding the atomic masses of all the atoms present in a molecule of the substance.

The molecular mass of Washing soda ${\mathrm{Na}}_2{\mathrm{CO}}_3.10{\mathrm{H}}_2\mathrm{O}$:

$$\begin{gathered} =2\times (\mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{sodium})+\mathrm{atomic}\mathrm{mass}\mathrm{of}\mathrm{carbon}+3\times (\mathrm{atomic}\mathrm{mass}\mathrm{of}\mathrm{oxygen})+20\times (\mathrm{atomic}\mathrm{mass}\mathrm{of}\mathrm{hydrogen})+10\times (\mathrm{atomic}\mathrm{mass}\mathrm{of}\mathrm{oxygen}) \\ =2\times 23+12+3\times 16+20\times 1+10\times 16 \\ =46+12+48+20+160 \\ =286\mathrm{a}.\mathrm{m}.\mathrm{u}. \end{gathered}$$Therefore, the molecular mass of Washing soda ${\mathrm{Na}}_2{\mathrm{CO}}_3.10{\mathrm{H}}_2\mathrm{O}$ is $286\text{a.m.u.}$