your instructor is correct; Kilogram is the SI unit od mass. If you do any physics work, you will see they use Kg nearly all of the time.

Chemists choose to use grams as their standard weight, because it is more convenient...although this often causes confusion in physical chemistry when you are doing physics equations, using kilograms, on chemical species, using moles and grams.

You got to keep alert on this!

Yes.
If you are wondering WHY the kilogram is the basic unit, and not the gram, the answer is
"well cos that's just what people decided!"

It is odd, because usually the basic unit has no prefix, and it's multiples of it which have prefixes.
But that's the way it is!

Personally, I blame the French.

Yeah, if they were being really pedantic they would have used the ton - a cubic metre of water. However, the kilogram is the most useful middle sized mass so they went with that. Most macroscopic objects in everyday life can be measured in kilograms, but few weigh over a ton.