2 SO _ 2 ( g ) + O _ 2 ( g ) → 2 SO _ 3 ( g ) The above reaction is...

$2{SO}_{2} ( g ) + {O}_{2} ( g ) \to 2{SO}_{3} ( g )$

The above reaction is carried out in a vessel starting with partial pressure ${P}_{{SO}_{2}} = 250mbar$, ${P}_{{O}_{2}} = 750mbar$ and ${P}_{{SO}_{3}} = 0bar$. When the reaction is complete, the total pressure in the reaction vessel is _____ $mbar$.

(Round off of the nearest integer).