4. Buffer: A. If you are asked to a 0.1 M buffer of pH-5, which one will you cho

Question

4. Buffer: A. If you are asked to a 0.1 M buffer of pH-5, which one will you chose from the following table?(5 pt) Answer Onionic Acid Table 4-7 Dissociation constant and pK, of some common wealk acids (proton donors) at 25 Acid K, (M) pk HCOOH (formie acid) CH COOH (acetic acid) CH&CH; COOH (propionic acid) CH CHIOH)COOH (lactic acid) HsPO4 (phosphoric acid HPO, (dihydrogen phosphate)138 x 10 HP (monohydrogen phosphate) H CO, (carbonic acid) HCOi (bicarbonate) NHi (ammonium) 3.75 4.76 4.87 3.86 2.14 6.86 12.4 3.77 10.2 1.78 x 10 1.74 × 10-5 1.35 × 10-5 1.38 10-4 7.25 x 103 3.98 x 10-B 1.70 x 10- 6.31 x 10- 5.62 x 10-10 9.25 B. If dihydrogen phosphate is available, what is the range of pH of buffers you can What is the pH of the solution made by mixing 50.0 mL of NaH2PO4 of 0.100 M of and 20.0 mL of H3PO4of 0.200 M. (10 pt) C.

Explanation / Answer

4. a) answer: propanoic acid (pka = 4.76)

b) H2PO4- , pka = 6.86

buffer pH range: (pka+1) to (pka-1)

      pH range = 7.86 -5.86

c) it is buffer

pH = pka + log(NaH2PO4/H3PO4)

    nO of mol of NaH2PO4 = 50*0.1 = 5 mmol

    no of mol of H3PO4 = 20*0.2 = 4 mmol

   pka of H2PO4- = 6.86

pH = 6.86 + log(5/4)

     = 6.957

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