In the reaction P + Q ® R + S. The time taken for 75% consumption of P is twice that of 50% consumption of P. The concentration of Q varies with reaction time as shown in figure. The overall order of the reaction is

Time required for 75% reaction of P is twice the time taken for 50% reaction, this indicates that the half life of the reaction is independent of the initial concentration and remains constant throughout the reaction.For first order kinetics, after first half life the concentration of P decreases up to 50%. After   half life the concentration of P once again decreases to its half value and now it reduced up to 25% of the initial concentration.Form the graph it is clear that reaction is zero order with respect to Q.R=k1P1Q0Overall order of the reaction = 1 + 0 = 1(Or)t75%=2t50%\first orderÞ order with respect to P is oneK=Q0−[Q]t⇒Q=−kt+Q0\ zero order ⇒ order with respect to Q is zeroOverall order = 0 + 1 = 1