Can someone explain me why the molar mass become 90.12 g/mol and when i calculated it's 90 g/mol?

It could be because of rounding error somewhere in your calculations. Another thing to note is 90 +/- 4 g/mol.

Your calculations don't have to be spot on.

pure water does not conduct electricity very well. however, when certain substances are dissolved in water, the solution does conduct electricity. you can make a simple device that shows how well a solution conducts electricity. this device uses a flashlight bulb to indicate how well the solution conducts electricity. the better the solution conducts electricity, the brighter the bulb will glow.

explanation:

the atomic mass is the bottom number on the element.

-steel jelly