What is the mass of 8.00 × 1022 molecules of nh3?

We find out the molar mass of NH₃.

atomic mass (N)=14 u
atomic mass (H)=1 u

atomic weight (NH₃)=14 u + 3(1u)=17 u

Therefore the molar mass of NH₃ will be: 17 g/mol
Then:
1 mol of water=18 g

2) We know that:
1 mol of any substance=6.022 * 10²³ particles.

3)We calculate the mass of 8.00 *10²² molecules

18 g of water6.022*10²³ molecules
x8.00*10²² molecules
 
x=(18 g of water * 8.00*10²²  molecules) / 6.022 * 10²³ molecules=2.39 g

the mass of 8.00 *10²² molecules of NH₃ would be 2.39 g.

Abbreviated as mg-25.