What is the conjugate base of H_2PO^-_4 ion? a) H_3PO_4 b) H_2PO^-_4 HPO_4^-2- d

Question

What is the conjugate base of H_2PO^-_4 ion? a) H_3PO_4 b) H_2PO^-_4 HPO_4^-2- d) PO_4^3- How many equivalence regions are observed in the titration of 0.00100M H_3PO_4 with strong base? pK_a's = 2.148, 7.198, 12.375 a) 1 b) 2 c) 3 d) 0, the concentration of weak acid is too dilute What is the pH of a 0.0100 M KH_2PO_4 solution? pK_a's above in #14 a) 2.00 b) 4.67 c) 9.79 d) 4.60 What is the pK_b of phosphate ion? pK_a's above in #14 a) 12.375 (b)1.625 c) 11.852 d) 6.802 What is the ionic strength of a 0.0100 M Na_2SO_4 solution? a) 0.0100 M b) 0.0600 M c) 0.0300M d) 0.0500 M Which is the charge balance for an aqueous Ca(NO_3)_2 solution? a) [Ca^2+] = [NO3^-] b) [Ca^2+] + [H^+] = [NO_3^-] + [OH^-] c) 2[Ca2^+] + [H^+ = [NO3^-] + [0H^-] Which of the following is/are true as you dilute a weak acid? a) % ionization increases b) % ionization decreases c) pH increases d) pH decreases e) both a and c

Explanation / Answer

14. answer is 3. three equivalence point will be observed as the acid contains three removable protons.

15. H2PO4^- + H2O <==> H3PO4 + OH-

Pkb of the above reaction : pKb = 14-pKa = 14-2.148 = 11.852

Kb = 10^-pKb = 10^-11.852 = 1.41*10^-12

Kb = X*X/0.01-X

as x is very small , 0.01-x = 0.01

Kb = x^2/0.01

or, 1.41*10^-12 = x^2/0.01

or, x = 1.19*10^-7 M

[OH-] = 1.19*10^-7 M,    [H+] = Kw/[OH-] = (1*10^-14)/(1.19*10^-7 M) = 8.43*10^-8 M

pH = 7.07

-------------------------------------

16.pKb = pKw -pKa

             = 14- 12.375 = 1.625

H2PO4^- H3PO4 OH- initial 0.01 0 0 change -X +X +X equilibrium 0.01-X X X

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