The decomposition of phosphine `(PH_(3))` on tungsten at low pressure is a first order reaction. It is because the

Explanation

`(t_(1))/(t_(2))=((a_(2))/(a_(1)))^(n-1)`
`a_(1)="290 mm of Hg "t_(1)="263 seconds"`
`a_(2)="360 mm of Hg "t_(2)="212 seconds"`
`(263)/(212)=((360)/(290))^(n-1)`
`1.24=(1.24)^(n-1)`
`n-1=1`
`n=1+1=2`
Hence, the reaction is of second order.