A pH or titration curve can be produced by plotting the pH of a solution (contai

Question

A pH or titration curve can be produced by plotting the pH of a solution (containing an analyte) versus the volume of added titrant. The equivalence or end point of a titration occurs when enough titrant has been added to consume all the acid or base initially present in the analyte. You know you have reached the endpoint when you see a significant change in the pH of the solution. When your analyte is a weak acid or base the titration, curve shows the effects of buffering before the equivalence point. The midpoint of the titration occurs in the buffer zone and is the point at which the pH is equal to the pK_a of the analyte. In the case of a weak acid-strong base titration the pH at the equivalence point will always be higher than 7. This is because of the basic properties of the anion (the conjugate base of the weak acid) that is present in the solution at this point. To the right is a titration curve for the titration of 25.0 mL of unknown compound X with 0.200 M NaOH. Use this titration curve to answer the following questions. What is the identity of compound X? What is the starting concentration of compound X?

Explanation / Answer

this is clearly a weak acid, since it forms a buffer

Vequivalence= 30 mL

so

half equivalence bvolume = 30/2 = 15 mL

the pH at V = 1 5mL = 4 approx

so..

pKa = 4-4.2 approx

V = 25 mL of X

M = NaOH; V = 30 mL

find identity of X...

so

mol of X = mol of base

mol of base = MV = 0.2*30 = 6 mmol of base

so

mmol of acid = 6 mmol

for

pKA = 4.20

this must be benzoic acid

b)

initial concentration

KA= [H+][A-]/[HA]

[H+] = 10^-2 = [A-]

10^-4.20 = (10^-2) (10^-2 ) / (M-10^-2 )

solve for M

M = (10^-2) (10^-2 ) / (10^-4.20 + 10^-2

M = 0.00993729 or 0.01 M approx

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