Please help fill in the white blanks and answer the 3 questions Introduction In

Question

Please help fill in the white blanks and answer the 3 questions

Introduction In this experiment you will titrate phosphoric acid (HzPO4, a triprotic weak acid) with sodium hydroxide NaOH, a strong base) to determine the initial concentration of the phosphoric acid and also the pK, for the second ionization of phosphoric acid. Phosphoric acid can exist in four distinct states of ionization (HzPO4, H2PO4, HPO42 and PO4), and each of these states is interconverted by the loss or addition of a proton The three equilibria that describe these states are defined by the following equations: Equation 2 HPOH+ HPO4 K-HHPO/[H,PO,] Equation 3 HPO42. H+ + PO43- Ko=R'IPO /[HPO Each of these equilibria is descrnbed by an acid disassociation constant (K,). Just as proton concentration can be quantified by pHlog(H], the acid disassociation constant can be quantified by pK. - -logK.. Values of pK, are useful because they can be directly related to values of pH. pH Titration Curve for a Triprotic Acid A pH titration curve relates the14 changes in pH of a solution of an acid or base as another solution of acid or base is added to it. In this 10 12 1PK, experiment a solution of sodium·1.p.. hydroxide will be added to a a solution of phosphoric acid. FigurepK 1 depicts a hypothetical pH. titration curve for the titration ofa 1eq 2 eq 3 ea solution of a triprotic acid with a 2 solution of sodium hydroxide as itrant 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Volume of Titrant (NaOH) mL Figure 1: pH titration curve for a triprotic acid

Explanation / Answer

1 )

H3PO4 + NaOH ----------------------> NaH2PO4 + H2O

2)

H3PO4 + 2 NaOH ---------------------> Na2HPO4 + 2 H2O

3 )

pKa1 = 2.15

pKa2 = 7.20

pKa3 = 12.35

first equivalence point =pH = (pKa1 + pKa2) / 2

= (2.15 + 7.20) / 2

= 4.67

second equivalence point =pH = (pKa2 + pKa3) / 2

= ( 7.20+12.35) / 2

   = 8.78

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