Posted by Anonymous on Sunday, April 19, 2009 at 10:56am.
For a weak acid, HA, being titrated with a strong base, such as NaOH, we have the equation,
HA + NaOH ==> NaA + H2O.
Now what is the Ka for the acid?
HA ==> H^+ + A^-
Ka = (H^+)(A^-)/(HA)
Let's solve for (H^+). That is
(H^+)=Ka*[(HA)/(A^-)]
Now let's turn our attention to the equation. Suppose we start with 2 moles HA. So when we get half way to the equivalence ponit (the equivalence point is when we have added 2 moles NaOH), we will have 1 mole HA left. Right? And how much NaA will have formed. Of course that's 1 mole has been formed because that's how much HA has been titrated. Now look at the equation. Plug in 1 for HA (that's whats left) and 1 for A^- (that's how much of the salt that's formed) and (H^+) = Ka. Neat, huh? It always works that way. When you are half-way to the equivalence point of a weak acid, (H^+) = Ka. When you are half-way to the equivalence point of a weak base, (OH^-) = Kb.
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