A buffer solution is prepared by mixing 89.3 mL of 0.815 M sodium dihydrogen cit

Question

A buffer solution is prepared by mixing 89.3 mL of 0.815 M sodium dihydrogen citrate with 53.0 mL of 0.0672 M sodium hydrogen citrate. A table of pKa values can be found here.


1. Calculate the pH (to two decimal places) of this solution.  

Assume the 5% approximation is valid and that the volumes are additive.

Incorrect. Tries 2/3 Previous Tries Table 10.2: pKa and pKb values at 25 C for some common weak acids and their conjugate bases Acid formula pKa Conjugate base Acid name Conjugate base name pKl CCOOH CCOO trifluoroacetic acid 0.20 trifluoroacetate ion 3.80 CI CCOOH 0.66 Cl3CCOO trichloroacetic acid trichloroacetate ion 3.34 HOOCCOOH 1.23 HOOCCOO oxalic acid hydrogen oxalate ion 12.77 dichloroacetic acid ClaHCCOOH 1.30 CI H CCOO dichloroacetate ion 12.70 sulfurous acid H2SO3 (HOSOOHj HSO. 1.77 hydrogen sulfite ion 12.23 hydrogen sulfate ion HSO, 1.92 SO sulfate ion 12.08 chlorous acid HCIO. 1.95 CIO. chlorite ion H PO. H PO phosphoric acid 2.12 hydrogen phosphate ion 11.88 chloroacetic acid CIH CCOOH 2.87 CIH CCOO chloroacetate ion 1.13 bromoacetic acid BrH CCOOH BrH CCOO bromoacetate ion 2.90 1.10 actic acid HE OCOOH 3.08 C2H5 ocoo lactate ion 0.92 2 H50 (COOH) 3.13 C3H50 (COOH) COOT dihydrogen citrate ion 0.87 citric acid 3 fluoride ion CaHTO COOH acetylsalicylate ion formate ion 5 7 4 COOH C5H7O4 COO ascorbic acid 4.10 ascorbate ion 9.90 hydrogen oxalate ion HOOCCOO OOCCOO 4.19 oxalate ion 9.81 4.20 C6Hscoo benzoic acid C6H5 COOH benzoate ion 9.80 C6H5NH 4.63 C6H5NH phenyl phenyl 9.37 ammonium ion amine (an ne

Explanation / Answer

Ans:- 1) Volume of sodium dihydrogen citrate =89.3ml = 0.0893L

Strength of sodium dihydrogen citrate =0.815M

Volume of sodium hydrogen citrate =53ml =0.053L

strength of sodium hydrogen citrate = 0.0672M

Total volume of buffer solution = 0.0893+0.053 = 0.1423L

Strength of sodium dihydrogen in buffer solution = 0.815 x 0.0893/0.1423

                                                                     = 0.511M

strength of sodium hydrogen citrate in buffer solution = 0.0672x0.053/0.1423

                                                                             = 0.025M

Now for dihydrogen citrate

pKb =10.87

Now by Henderson Hasselbach equation

pOH =pKb + log([sodium dihydrogen citrate ] /[sodium hydrogen citrate])

        =10.87+log([0.511]/[0.025])

         = 10.87+1.31

         = 12.18

Now

pH= 14-pOH

    =14-12.18

    = 1.82

Ans 2) Now when we add 6.21g of sodium hydrogen citrate

strength of sodium hydrogen citrate = 6.21/53

                                                    = 0.117M

Now strength of sodium hydrogen citrate in the buffer solution = 0.117 x 0.053 /0.1423

                                                                                           = 0.043M

Now pKb= 10.87

pOH= pKb+log[sodium dihydrogen citrate / sodium hydrogen citrate]

      =10.87+log(0.511/0.043)

       = 10.87+1.07

       = 11.94

Now pH=14 - pOH

           =14-11.94

            = 2.06

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