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Problem: weak acids

  1. Jun 8, 2005 #1


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    hi forum,
    would nebody be able to help me solve this problem?

    0.18mol of potassium cyanide (KCN) was dissolved in 1.00L of a solution in which the pH was held constant at 9.70 at a temperature of 298K.

    the equilibrium concentration of CN- was 0.13M.

    Calculate Ka for the weak acid HCN.

    ne help would be greatly appreciated. :)
  2. jcsd
  3. Jun 8, 2005 #2


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    Staff: Mentor

    Use Henderson-Hassebalch equation. It wont be difficult to calculate equilibrium concentrations of HCN and CN- - just enter them into HH and solve for pKa.
  4. Jun 8, 2005 #3


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    Science Advisor
    Homework Helper

    Start with finding Kb

    [tex]Kb= \frac{[OH-][HCN]}{[CN-]} [/tex]

    You can find the initial concentration, you also know the equilibrium concentration, thus you can find how much of CN- reacted. It also says that the pH was held constant. From the pH given, find the pOH, from this calculate the concentration of hydroxide [OH-]. This is the equilibrium concentration thus you can plug it back into the equilibrium equation.

    How would you find the HCN concentration to solve for Kb...and then Ka?
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