Sketch titration curves showing the key points for the following reactions and b

ID: 934593 • Letter: S

Question

Sketch titration curves showing the key points for the following reactions and be able to explain your reasoning showing necessary calculations

(a) A 10-mL aliquot of 0.100 M sulfurous acid, H2SO3, titrated with 0.100 M NaOH. Acid Dissociation Constants for sulfurous acid: pKa1 = 1.82; pKa2 = 7.20

(b) A 10-mL aliquot of 0.0500 M Na3A (A=protocatechuic acid) titrated with 0.100 M HCl. Acid Dissociation Constants for H3A: pKa1 = 4.28; pKa2 = 8.81; pKa3 = 13.02

Sketching the curve doesn't seem to be a big problem. But I am very confused with the calculations. I am using m1v1=m2v2 to find the volume of the euqivalence point. But how would I find the pH of the equivalence points and half equivalence points?

Explanation / Answer

H2SO3 is a diprotic acid H2A, it will be 2 EP at 10 and 20 mL

V, mL

Relevance

How to calculate pH

(solution contains..........)

Formula

start

Weak acid H2A

pH = 1/2 pKa1 -1/2 logcacid

= 0.91 -0.5(-1) = 1.41

Half/semi EP

Buffer H2A/HA-

pH = pKa1 = 1.82

First EP

Ampholite HA-

pH = (pKa1 + pKa2)/2 = 4.51

Second Half EP

Buffer HA-/A2-

pH = pKa2 = 7.2

Second EP

Weak base A2-

pOH = 1/2 pKb2 -1/2 logcbase = 6.8/2