a gaseous reaction, A2(g)-----B(g)+1/2C(g) shows increase in pressure from 100mm to 120mm in 5 minutes.The rate of disappearance of A2 is-

The equation for pressure: A₂ (g) -->  B (g) + 1/2 C(g)
Here, the pressure is , initially 100  .... 0...........0
At reaction time,pressure ,100-x .....,,..x ...... 1/2x 
So, we knwo that,
Change in pressure, 100+ 1/2 X = 120
So, X = 40 mm
Hence, the rate of disappearance of A₂ is, -d[A₂]/ dt 
Here, change in time dt = 5 min
So, -d[A₂]/ dt = 40/ 5 = 8mm/min