Which of the following combinations would be best to buffer an aqueous solution

Question

Which of the following combinations would be best to buffer an aqueous solution at a pH of 7.0? a. H_3PO_4 and H_3PO^-_4 K_a1 = 7.5 times 10^-3 b. HNO_2 and NO^-_2, K_a = 4.5 times 10^-5 c. CH_3CO_2H and CH_3COO^-, K_a = 1.8 times 10^-5 d. H_3PO^2-_4, K_a = 6.2 times 10^-3 NH^+_4 and NH_3, K_a = 5.7 times 106-20 When mixed in appropriate amounts, each of the following mixtures can produce an effective buffer solution EXCEPT a HCl and NaH_3PO4. b. Na_3HPO_4 and Na_3PO_4. c. Na_3HCO_3 and Na_2CO_3 d. NaH_3PO_4 and Na_3HPO_4. e,. NaOH and NaF All of the following concerning acid-base buffers are true EXCEPT a buffers are resistant to pH changes upon addition of small quantities of strong acids or bases. b. buffers are used as colored indicators acid-base titrations. c the pH of a buffer is close to the pK_a of the weak acid from which it is made. d. buffers contain appreciable quantities of a weak acid and its conjugate base. e buffers are resistant to changes in pH when diluted with water.

Explanation / Answer

4) For any good buffer the Pka value will be almost close to the PH of the aqueous solution,
   Pka = -log ka
   So the answer will be option d) H2PO4- and HPO42- whose Pka = -log(6.2*10-8) = 7.2 ( almost near to Ph =7)
5) Option e) NaOH and NaF is the answer.
6) Buffers are not used as colored indicators in acid-base titrations. So all statements are true except option b)
  

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