Weak Acid Equivalence Point

Name: Jesse
Status: educator
Grade: 9-12
Location: WA

Question: Why is the equivalence point for titration of weak acids
above pH 7. I know that it is has something to do with the conjugate
bases attracting protons, but I still do not understand how the
moles of OH- equal the moles of H+ in a basic solution.
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It is not.  The equivalence point for titration of a weak acid is when 
the pH equals the pKa of the acid.  The equivalence point is not when 
[H+] = [OH-]; it is when [HA] = [A-].

Richard Barrans
Department of Physics and Astronomy
University of Wyoming
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Jesse,
 
It is important to remember that the salt produced from a weak acid 
and a strong base will hydrolyze (react with water) and produce a base.
 
Let "HA" represent the proton and the counter anion in a weak acid, let 
"MOH" represent a strong base (a metal such as Na along with the hydroxide, 
OH) and we can represent a general weak acid and strong base reaction as:
 
HA + MOH = MA + H2O
 
but the salt (of the weak acid) will fully dissociate to form:
 
MA = M(+) + A(-)
 
The M(+) will not hydrolyze (doing so will produce strong, higher energy, 
substances), but the A(-) can react with water in an equilibrium as represented 
by:
 
A(-) + H2O = HA + OH(-)
 
Note that a base, OH(-) is formed in the hydrolysis of the anion of the weak 
acid. Thus, although an equivalent amount of base is used to react with the 
weak acid (thereby neutralizing the acid and causing your expectation of a 
pH = 7 at the equivalence point), a base is produced in the process. Thus at 
the equivalence point, the pH is that of a base.
 
Greg (Roberto Gregorius)
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   First, let udistinguish between terms that are often confused "dilute 
   vs.concentrated" and "weak vs.strong".


Dilute and concentrated refer to the AMOUNT of acid present in the solution. 
Weak and strong refer to the degree to which the acid is ionized in solution.
   A "strong acid" is an acid that ionizes completely in water. HCl is an 
   example: HCl ---> H(+1) + Cl(-1).
There is essentially NO concentration of acid present in the neutral form 
(HCl) dissolved in the water.
   A "weak acid" is an acid that only ionizes partially in water. This can 
   be written in a "generic" form:
HA ---> H(+1) + A(-1). So in addition to  H(+1) and A(-1) in solution, there 
is also a measurable amount of
neutral, molecular HA present.
   The relative amount of the three species is expressed by an "equilibrium 
   constant", or "ionization constant" --
both terms are used. The equilibrium constant is defined as: Keq = (H+)x(A-) / (HA), 
where the terms in parentheses are the molar concentration (mols / liter) of 
each of the species. This Keq is constant at a fixed temperature.
   You can look at (HA) as a "storage bin" of available (H+). As you begin to 
   titrate the "weak acid" with base, (BOH), you convert (H+) into H2O. This 
   removes some (H+). But the Keq, which is a constant, must be maintained. 
   This ratio is kept fixed by ionizing an equivalent amount of molecular (HA) 
   into (H+) and (A-).
If you titrated the solution to pH = 7 you would only have consumed the amount 
of (H+) present as ions. You have to add more base to react the amount of (H+) 
that has been "stored" in the form of molecular, neutral (HA) until that amount 
of (H+) has also been titrated. That means adding more base. You have to add the 
amount of base until all of the "stored" (H+) has been neutralized. The amount of 
base required to do this is controlled by the value of Keq which determines the 
relative amount of (H+) and (HA). The result of having to add this "extra" base is 
to make the equivalence point greater than pH=7.
     To summarize: (H+) in a solution of a "weak" acid is present in two forms: (H+) 
	 and (HA). You have to add sufficient base (BOH) to neutralize BOTH forms in which 
	 (H+) is present, because as you consume (H+) by neutralization, the neutral form 
	 (HA) will ionize more to maintain the ratio of (H+) to (HA) governed by Keq.
	 
Vince Calder
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