Formula unit mass for ${\mathrm{K}}_2{\mathrm{CO}}_3$ is

Analyze atomic weights of elements in Potassium Carbonate ( ${\mathrm{K}}_2{\mathrm{CO}}_3$)

• The atomic mass of Potassium (K)$=39\mathrm{u}$

So, for two K atoms, atomic mass = $2\times 39=78\mathrm{u}$

• The atomic mass of Carbon (C) $=12\mathrm{u}$

So, for one C atom, atomic mass = $1\times 12=12\mathrm{u}$

• The atomic mass of Oxygen (O)$=16\mathrm{u}$

So, for three O atoms, atomic mass = $3\times 16=48\mathrm{u}$

Analyze formula unit mass

Formula unit mass for ${\mathrm{K}}_2{\mathrm{CO}}_3$ is:

${\mathrm{K}}_2{\mathrm{CO}}_3=(2\times \mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{K})+(1\times \mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{C})+(3\times \mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{O})$

$=78+12+48=138\mathrm{u}$

Hence, the formula unit mass for ${\mathrm{K}}_2{\mathrm{CO}}_3$ is $138\mathrm{u}$