Answer:
Since the pKb is is 4.75, the pKa is 9.25. Also, in the Henderson Hassalbalch, Molarity and moles are generally interchangeable if its the same solution.
pH = pKa + log (NH3/NH4Cl)
9.05 = 9.25 + log (Mole Ratio)
-0.2 = log (Mole Ratio)
10^-0.2 = Mole Ratio
Mole Ratio = 0.631
Now, that means for every mole of Acid, there are 0.631 moles of base. So we multiply the moles of base by the molar mass of NH3 and we multiply the moles of Acid by the molar mass of NH4Cl.
Base: 0.631 mol x 17.0 g/mol = 10.7 g NH3
Acid: 1.00 mol x 53.5 g/mol = 53.5 g NH4Cl
Now we just divide them to get the ratio.
10.7 g NH3 / 53.5 g NH4Cl = 0.200
