Ka Value

[RETake09]

Question asks: Write out the chemical equation, determine the hydrogen ion concentration ([H+]; [H3O+]); the hydroxide ion concentration ([OH-]), the pH and the pOH of the following solutions. The Ka value acid is given in parentheses. Use HA to represent the acid. All acids have only one ionizable H.

0.10 M HC2H302; Ka=1.76x10^-5

[kyle1990]

The dissociation of a general acid is often written as

HA ----> H+   +   A-


From there, write out an ICE chart beneath your chemical equation. Since 100*[HA] is much less than the value of Ka, we can assume that acetic acid dissociates to a very slight extent and we can thus simplify the math.

               HA------> H+    + A-
          I    0.10 M      0         0
         C    -x            +x       +x
         E    ~0.10        x        x

Write out the equilibrium expression for the reaction. Plug in the equilibrium values (in red) into the expression and solve for x. X represents the concentration of hydronium ions and the concentration of A-. Now that you know the hydronium ion concentration, you can find the pH, pOH, and [OH-]