$-\mathrm{NH}_{2}+\mathrm{CH}_{3} \mathrm{COCl} \longrightarrow-\mathrm{NHCOCH}_{3}+\mathrm{HCl}$

The above equation suggests that, the acylation of each $-\mathrm{NH}_{2}$ group increases the mass by $42 \,\mu$ $[43-1] .$ If the molecular mass of the organic compound is $180 \,\mu$ while that of the acylated product is $390 \,\mu$, then the increase in the mass due to acylation is given by

$390-180=210 \,\mu$

Hence, the number of $-\mathrm{NH}_{2}$ groups $=\frac{210\, \mu}{42\, \mu}=5$