Thymol blue is chosen as the indicator for the titration of citric acid. The col

Question

Thymol blue is chosen as the indicator for the titration of citric acid. The color change from yellow to blue occurs at a pH = 8.6. The concentration of citric acid is 0.064 M in a 25 mL total volume. It is tritrated with 0.064 M NaOH. For citric acid, K1 = 7.4 Times 10-4, K2 = 1.7 times 10-5, K3 = 4.0 Times 10-7. What is the pH of the solution at the third equivalence point of the titration? Check your approximations. What is the equilibrium concentrations of Hci at the end point? What is the analytical concentration of HCi at the end point? What is the titration error associated with using thymol blue for the citric acid titration? Don't check your approximations.

Explanation / Answer

Concentration of the salt formed at the end of third equivalence point = 0.064/3

=0.021333 (since three moles of NaOH reacts with one mole of HCi hence I divided it by 3)

Now since this is a salt of a strong base and a weak acid,

pH=7+0.5pKa+0.5log(C) (where C is the concentration of the salt formed).

now pKa=pKa3 = 6.4

so, pH=7+0.5*6.4+0.5*log(0.02133)

=9.36

b)equilibrium concentration of HCi = [H+][Ci-]

=0.02133*10^-9.36

=9.31*10^-12

c)analytical concentration=0 M

d) Titration error = 9.31*10^-12 =[HCi]

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