Question

A solution is 0.10M in aqueous NH3. Calculate (a) the number of moles and (b) the number of grams of NH4Cl that must be added to 500mL of this solution to prepare a buffer solution with pH=9.15. You may neglect the volume change due to addition of solid NH4Cl. Kb=1.8x10^-5.
(atomic wts: N=14 H=1 Cl=35.45)

Answer 1

pH = pKa + log (base)/(acid)
You know pH, Ka is calculated from KaKb = Kw = 1E-14 and pKa = -log Ka.
You know base is 0.1, substitute into the above and solve for acid. Since the problem asks for mols and grams, the easy to do this is to substitute millimols into (base) and (acid) instead of concentrations so the unknown is millimols acid. Converted to mols that will give you part a. Then mols x molar mass will give you part b.
Concentration and mols is not the same but the math comes out to be the same answer so the shortcut saves sine time.