For  ${\mathrm{N}}_2+3{\mathrm{H}}_2⟶2{\mathrm{NH}}_3$ , rates of disappearance of N₂ and H₂ and rate of appearance of NH₃ respectively are a, b and c, then

Rate of dissappearance of N₂ = $-\frac{\mathrm{d}\left[{\mathrm{N}}_2\right]}{\mathrm{dt}}$

Rate of dissappearance of H₂ = $-\frac{1}{3}\frac{\mathrm{d}\left[{\mathrm{H}}_2\right]}{\mathrm{dt}}$

Rate of appearance of NH₃ = $\frac{1}{2}\frac{\mathrm{d}\left[{\mathrm{NH}}_3\right]}{\mathrm{dt}}$

For 1 mole of N₂ disappearing 3 moles of H₂ is disadppearing and 2 moles of NH₃ is appearing