How many molecules of nh3 are produced from 4.72x10 negative 4 power g of h2?

9.40x10^19 molecules.
   The balanced equation for ammonia is:
 N2 + 3H2 ==> 2NH3
   So for every 3 moles of hydrogen gas, 2 moles of ammonia is produced. So let's calculate the molar mass of hydrogen and ammonia, starting with the respective atomic weights:
 Atomic weight nitrogen = 14.0067
  Atomic weight hydrogen = 1.00794

   Molar mass H2 = 2 * 1.00794 = 2.01588 g/mol
 Molar mass NH3 = 14.0067 + 3 * 1.00794 = 17.03052 g/mol

   Moles H2 = 4.72 x 10^-4 g / 2.01588 g/mol = 2.34140921086573x10^-4 mol
   Moles NH3 = 2.34140921086573x10^-4 mol * (2/3) = 1.56094x10^-4 mol

Now to convert from moles to molecules, just multiply by Avogadro's number: 1.56094x10^-4 * 6.0221409x10^23 = 9.400197448261x10^19
   Rounding to 3 significant figures gives 9.40x10^19 molecules.