The scale in this puzzle gives you an actual weight.
In a more normal situation, you'd probably weigh a pill
from each jar until you found one that weighed 9 grams.
That won't work here, since you've got only one
Measurement.

You need to manufacture a situation in which the
weight measurement is informative. One solution is to call
the bottles #1, #2, #3, #4, and #5. Take one pill from #1, two
from #2, three from #3, four from #4, and five from #5.
Weigh the whole lot. Were all the pills normal, the result
would be 10 + 20 + 30 + 40 + 50 = 150 grams. In fact, the
weight must fall short of this by a number of grams equal to
the number of the contaminated bottle. Should the total
weight be 146 (4 grams short), then bottle #4 must contain
the lighter pills.

**Another solution:**
An alternate solution, with the arguable virtue of
weighing fewer pills, is to weigh 1 + 2 + 3 + 4 pills from the
first four bottles only. Then, if the weight is less than 100
grams, the shortfall amount points to the rogue bottle.
Should the measurement be 100 grams on the nose, the fifth
bottle must be contaminated.