The phosphoric acid in a 100.00-mL sample of cola drink was titrated with 0.1025
ID: 498571 • Letter: T
Question
The phosphoric acid in a 100.00-mL sample of cola drink was titrated with 0.1025 N NaOH. If the first equivalence point occurred after 13.11 mL of base was added, and the second equivalence point occurred after 28.55 mL of base, calculate: 1. The concentrations or molarity of H_3PO_4 {H_3PO_4^- + OH^- rightarrow H_2PO_4 + H_2O} The concentration of H_2PO_4 in the cola sample {H_2PO_4^- + OH^- rightarrow HPO^-2_4 _ H_2O) How would CO_2 interfere with the titration of H_3PO_4? {CO_2(g) + H_2O rightarrow H_2CO_3} a) Consider the dissociation of triprotic acid, H_3Y: H_3Y_(aq) + H_2O_(l) H_2Y^-_(aq) + H_30^+_(aq) K_a1 = 2.0 Times 10^-4 H_2A_(aq) + H_2O_(l) HY^2-_(aq) + H_30^+_(aq) K_a2 = 8.0 Times 10^-8 HY^2-_(aq) + H_2O_(l) Y^2-_(aq) + H_3O^+_(aq) K_a3 = 4.0 Times 10^-13 2H_2O_(l) H_2O^+ + OH^- K_w = [H_3O^+][OH^-] = 1.0 times 10^-14 the values of K_a1, K_a2, K_a3, and K_w and evaluate the equilibrium constant (K_eq) associated with the three reactions below. with K_a1 and K_w and determine a numerical value for K_eq1. Repeat the same calculation and determine a numerical value for K_eq2 using the numerical values of K_a2 and K_w. Repeat the same calculation and determine a numerical value for K_eq3 using the numerical value K_w. H_3Y + OH^- doubleheadarrow H_2Y^- + H_2O_(l) K_eq1 =Explanation / Answer
2. The volume of NaOH required to reach 2nd equivalence point = 28.55 - 13.11 = 15.44 mL = 15.44 X 10-3 L
Concentration of NaOH = 0.1025 N
moles of NaOH (n) = 0.1025 X 15.44 X 10-3 = 1.58 X 10-3 mol
since the reaction is 1:1 the number of moles of NaH2PO4 will be 1.58 X 10-3 mol
The concentration of NaH2PO4 in the cola sample = number of moles of NaH2PO4 / Volume of the cola
= 1.58 X 10-3 mol / 100 X 10-3 L
= 0.0158 mol L-1
3. CO2 gas reacts with water to give H2CO3. H2CO3 is acidic and can be neutralised by NaOH thus giving erroneous results. These type of titrations are usually done while monitoring the pH of the system. Presence of H2CO3 in addition to H3PO4 could also affect the pH readings. A way to get rid of CO2 and H2CO3 is by boiling the cola for 30 min.
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