Question

how many molecules (not moles) of nh3 are produced from 6.62×10−4 g of h2?

Answer 1

N₂(g) + 3 H₂(g) = 2 NH₃(g)       molar mass N₂ = 28.0 g/mol
                                                                     NH₃ = 17.0 g/mol

14 g N₂ ---------------> 2 x 17 g NH₃
6.62 x 10⁻⁴ g --------> ?

Mass NH₃ = ( 6.62 x 10⁻⁴ ) x 2 x 17 / 14

Mass NH₃ = 0.022508 / 14

Mass NH₃ = 0.00160 g

number of moles = 0.00160 / 17 =  9.41 x 10⁻⁵ moles

1 mole ----------------------- 6.02 x 10²³ molecules
9.41 x 10⁻⁵ moles ----------  ?

molecules NH₃ =  ( 9.41 x 10⁻⁵ ) x ( 6.02 x 10²³ ) / 1

= 5.66 x 10¹⁹  molecules of NH₃

hope this helps!