Question

Using Henderson Hasselbach, calculate the pH of a buffer that has 0.65M ammonia and 0.50 M ammonium nitrate before and after 0.06 M nitric acid is added. Write the equilibrium and buffering reactions. Kb = 1.8 x 10 -5 (show all reactions and work, with units)

Answer 1

Henderson-hasselblach's equation is

pOH = pKb + log [salt]/[base]

pOH = -logKb + log [salt]/[base]

Before addition of HNO3:

pOH = -logKb + log [NH4NO3]/ [NH3]

= - log(1.8 x 10^-5) + log (0.5/0.65)

= 4.63

pOH = 4.63

pH = 14 - pOH = 14-4.63 = 9.37

Therefore,

pH before addition of HNO3 = 9.37

After addition of 0.06M HNO3:

   NH3 + HNO3 ----------> NH4HNO3

0.65 M 0.06 M 0

-----------------------------------------------------------

0.65-0.06 M 0 0.06 M

= 0.59 M

Hence,

[NH3] = 0.59 M

[NH4NO3] = 0.50 M + 0.06 M = 0.56 M

pOH = -logKb + log [NH4NO3]/ [NH3]

= - log(1.8 x 10^-5) + log (0.56/0.59)

= 4.72

pOH = 4.72

pH = 14 - pOH = 14-4.72 = 9.28

Therefore,

pH before addition of 0.06 M HNO3 = 9.28