Consider the reaction, Ag  →   Bg  + Cg. Initial pressure of ‘A’  is p0mm. After time ‘t’, total pressure is pt. If the reaction follows first order kinetics, rate constant is given by the expression

Ag →   Bg  + Cg Initial pressure          p0            -      - Pressure after ‘t’    p0-x         x        x ∴     pt=p0 − x+x+x pt=p0+x x=pt−p0;   K=2.303t   log  p0p0−x;     K=2.303t   log  p0p0−pt−p0 K=2.303t   log  p02p0−pt