In the formation of sulphur trioxide by the contact process,

$2{\mathrm{SO}}_2\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\rightleftharpoons 2{\mathrm{SO}}_3\left(\mathrm{g}\right)$

The rate of reaction is expressed as

$-\frac{\mathrm{d}\left({\mathrm{O}}_2\right)}{\mathrm{dt}}=2.5\times {10}^{-4}{\mathrm{molL}}^{-1}{\mathrm{s}}^{-1}$

The rate of disappearance of (SO₂) will be