Label the five jars A,B,C,D, and E. Then take one pill out of jar A, two pills from jar B, three from jar C, and four from jar D.
Weigh the pile of pills (1A+2B+3C+4D).
If the weight is 100 then the lighter pills are in jar E. If the weight is lighter then:
If the weight is 99 it is in A.
If the weight is 98 it is in B.
If the weight is 97 it is in C.
If the weight is 96 it is in D.