An oil drop has 7.99 × 10−19 C charge. How many electrons does this oil drop has?
5
Just recall Millikan oil drop experiment.
From that, we can say
Charge on the electron = 1.6 × 10−19C
Total charge on the oil drop = 7.99 × 10−19C
→ If there are no electrons on oil drop, there should be no charge.
Hence, the number of electrons = 7.99×10−191.6×10−19 = 5
Therefore there are 5 electrons on the oil drop.