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Citric acid is a triprotic acid (H3L), with acidity constants

Ka1 = 10-3.13, Ka2 = 10-4.72, and Ka3 = 10-6.33.  

(1) Construct a logC-pH diagram using Excel or other similar software to illustrate the speciation of the solution containing 1.0´10-3 M of total citric acid. Based upon the speciation diagram, determine the most dominant species at pH 4 and pH 9, respectively.


(2) A solution is prepared by adding 1.0´10-3 M trisodium citrate (Na3L) to pure water. Calculate the equilibrium pH of this solution.


(3) Solve the question (b) using graphical method.


(4) Calculate the equilibrium pH for a solution that contains 1.0´10-3 M trisodium citrate (Na3L) and 0.10 M of CaCl2. (note: you must consider the deviation of species from perfect activity in this problem, and include activity correction in your calculation)

(5) Orange juice consists of citric acid and non-acid/base ionic species, and thus orange juice's acid-base behavior can be modeled by citric acid. An orange juice was measured to be at pH 4.0. Alkaline titration on this orange juice showed that adding 3.56´10-2 M of NaOH raised the pH to 7.0. Use the above information to calculate the concentration of total citric acid in this orange juice. Assume dilution is negligible during the titration. (Hint: Although you do not know the entire composition of the orange juice, you can determine the contribution of background inert species to ENE.) 

(7) Boric acid (H3BO3) is a weak monoprotic acid. Boric acid is a commonly used buffer for alkaline pH because of its inertness in many chemical reactions. In the lab, you wish to maintain a reactor's solution at constant pH of 9.0, so that the pH does not vary by more than 0.1 unit pH upon addition of 1.0×10-3 M of NaOH or HCl. You plan to make the buffer by adding X M of H3BO3 and Y M of NaOH. Determine the values of X and Y.  

H3BO3 + H2O = B(OH)4+ H+ Ka = 10-9.22  


(1) A solution consists of 2.0´10-3 M NaHCO3. Use Excel or other similar software to plot a titration curve of this solution by strong NaOH and HCl, i.e., a pH vs. CB-CA plot. Mark the equivalence points (f = 0, 1, and 2) on the curve.

(2) What is the buffer intensity of this solution at pH 7?

(3) Calculate the alkalinity (in units of both eq/L and mg/L as CaCO3) of this solution.

(4) Calculate the new alkalinity if this solution is added with each of the following chemicals (as three separate scenarios):

(i)        1.0´10-3 M NaOH

(ii)       1.0´10-3 M H2SO4

(iii)     1.0´10-3 M NH4Cl

Relevant equilibrium constants

H2O = H+ + OH- Kw = -14.0

H2CO3* = HCO3- + H+ Ka1 = -6.35

HCO3- = CO32- + H+ Ka2 = -10.33

NH4+ = NH3(aq) + H+ Ka= = -9.25

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