easy for a 100mM solution you want:
MW x 0.1 x V

with V the volume of the solution you want in liter
and MW the molecular weight of your compound.

If you need to make a solution with the concentration of 100mM you need the molecular weight of 79g/mol of substance and dilute to a liter you need 7.9grams of substance mixed with 1L of water to make a 100mM solution or as your collegue said 4grams in a 50ml of water will work as well

Just don’t forget the implied units. Writing 100 mM as 0.1 means 0.1 mole/L. So V has to be in L (50 ml = 0.05 L) or you won’t get the correct answer–those pesky orders of magnitude again, but for different reasons. For the anhydrous salt it works out to 0.4 g rounded off to the nearest tenth of a gram.

Hmm. I get 79 g/mole x 0.1 mole/L x 0.05 L = 0.395 g or 0.4 g of anyhdrous ammonium bicarbonate–unless I’m doing something I ortent.

I said to put the volume in liters. 😉
And I really suggest the original poster to have a look back at those Chemistry 101 lessons on the dilutions etc. Because that is something that will be useful all the time in biology, and you cannot always rely on other people for such basic calculations.