The rate of conversion for a reaction occurring in a closed system is defined as the time derivative of the extent of reaction: $$\stackrel{\text{.}}{\xi }=\frac{\mathrm{d}\xi }{\mathrm{d}t}$$ In view of the definition of extent of reaction it follows that with reference to any species in a reaction showing time-independent stoichiometry $$\stackrel{\text{.}}{\xi }=\frac{\mathrm{d}\xi }{\mathrm{d}t}=\frac{1}{{\nu }_i}\frac{\mathrm{d}{n}_i}{\mathrm{d}t}$$ where $n_i$ is the amount of the species at any time and ${\nu }_i$ is its stoichiometric coefficient.