How many molecules are in 5.00 grams of NH3?

you will need to work out how many moles are in NH3, to get this you divide the mass by NH3's RMM (relative molecular formula) [14 + (3x1) = 17]

5.00/17=0.294 moles

so now that you've got the moles to get molecules you need to times the moles by avargado's constant 6.022x10^23

the rule is that one mole of compound containes 6.022x10^23 molecules

so now you times your moles of NH3 (0.294) by avargados constant

0.294 x 6.002x10^23 = 1.765x10^23

just to clarify thats 1.765 times ten to the power of 23