What is a converging lens

Calculate the molar mass of aluminum oxide (al2o3). express your answer to four significant figures.

How to convert a density of 0.791g/cm3 to lb/ft3

In the millikan oil drop experiment they determined that every drop had a charge which was a while number multiple of -1.60x10^-19. if a drop has a total charge of -9.60x10^-19 then how many excess electrons are contained within the drop?