Chemistry Question

Question

Suppose 35 mL of 0.1 M of ammonia, NH3 is moxed with 20 mL of 0.1 M of HCl. Calculate the pH of the resulting solution.

Answer 1

This is a buffer problem.  Adding HCl to NH3 produces NH4Cl.  Thus, you now have a solution of a weak base (NH3) and the salt of that base (NH4Cl), thus making a buffer.  You need to know either the Kb for NH3 or the Ka for NH4+ to answer this.  You can then use either pH = pKa + log [salt]/[acid] or pOH = pKb + log [salt]/[base].  Understand that KaKb = Kw = 1x10^-14 and also recognize the hydrolysis of NH4Cl:  NH4+ + H2O ==> NH3 + H3O+ and Ka = [NH3][H3O+]/[NH4⁺]

0.035 L x 0.1 mol/L = 0.0035 moles NH3

0.020 L x 0.1 mol/L = 0.0020 mole HCl

moles NH4Cl = 0.0020

moles NH3 = 0.0015

Final Volume = 55 ml = 0.055 L

Final [NH4+] = 0.0020 mol/0.055 L = 0.0364 M

Final [NH3] = 0.0015 mol/0.055 L = 0.0273 M

Use which ever form of Henderson Hasselbalch you wish, with the appropriate Ka or Kb values and solve for pH