October 24, 2021, 01:48:47 AM
Forum Rules: Read This Before Posting


Topic: pH of NH4CN solution  (Read 18957 times)

0 Members and 1 Guest are viewing this topic.

Offline Rutherford

  • Sr. Member
  • *****
  • Posts: 1868
  • Mole Snacks: +60/-29
  • Gender: Male
pH of NH4CN solution
« on: June 16, 2012, 10:46:53 AM »
Calculate the pH of a 0.7M solution of NH4CN. KbNH3=1.79*10-5, KaHCN=7.9*10-10.

The reactions are:
NH4++H2O ::equil:: NH3+H3O+ Ka=Kh=Kw/Kb=5.59*10-10
0.7                  x      x
CN-+H2O ::equil:: HCN+OH- Kb=Kw/Ka=1.27*10-5
0.7                y     y

[H+]=x=1.98*10-5
[OH-]=y=2.98*10-3
There are more OH- ions :rarrow: y-x≈2.98*10-3
pOH=2.52
pH=14-2.52=11.48 but in my book it is 9.18.
What did I do wrong?
« Last Edit: June 16, 2012, 11:13:27 AM by Raderford »

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 26800
  • Mole Snacks: +1737/-403
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: pH of NH4CN solution
« Reply #1 on: June 16, 2012, 11:59:03 AM »
You can't assume there are two different, separate reactions taking place. You ended with OH- being neutralized by H+ - once they react, they are removed from the solution, so the equilibrium shifts.

http://www.chembuddy.com/?left=pH-calculation&right=pH-salt-simplified
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Offline Rutherford

  • Sr. Member
  • *****
  • Posts: 1868
  • Mole Snacks: +60/-29
  • Gender: Male
Re: pH of NH4CN solution
« Reply #2 on: June 16, 2012, 12:52:02 PM »
Understood, thanks.

Sponsored Links