Titration of Weak Base with Strong Acid

10-2 Titration of Weak Acid with Strong Base 

The titration of a weak acid with a strong base puts all our knowledge of acid-base chemistry to work. The example we treat is the titration of 50.00 mL of 0.020 00 M MES with 0.100 0 M NaOH. MES is an abbreviation for 2-(A -morpholino)ethane-sulfonic acid, a weak acid with pK = 6.27. It is widely used in biochemistry as a buffer for the pH 6 region. 

It is helpful first to calculate the volume of base needed to reach the equivalence point. Because 1 mol of OH reacts with 1 mol of MES, we can say 

The titration calculations for this problem are of four types  

Before any base is added, the solution contains just HA in water. This is a weak-acid problem in which the pH is determined by the equilibrium 

Reaction 11-2 is the reverse of the Kb reaction for the base A. Therefore, the equilibrium constant for Reaction 11-2 is K = /Kh = /(Kw/Ka (for HA)) = 5.4 X 107. The equilibrium constant is so large that we can say that the reaction goes to completion after each addition of OH-. As we saw in Box 9-3, strong plus weak react completely. 

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Titration of Weak Acids with Strong Bases... 

I B I 11-2 Titration of Weak Acid with Strong Base... 

Titration of weak acid with strong base ... 

O Effect of pKa in the titration of weak acid with strong base. Use Equation 11-9 with a spreadsheet such as the one shown in Figure 11-11 to compute and plot the family of curves at the left side of Figure 11-3. For a strong acid, choose a large A"a, such as Ka = 102 or pKa = -2. 

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