Question

9.The pka of NH is 9.25. What is the highest pH that a buffer system composed of NH4 and NH3 will buffer at effectively? a) 13.25 b) 12.25 c) 11.25 d) 10.25 Which of the following mixtures is a buffer: a) 25mL 0.1M HF b) 25mL 0.1M HCI mixed with 25mL 0.1M NaOH. c) 25mL 0.1M NH3 mixed with 12.5mL 0.1M HCI. d) 25mL 0.1M NaF 11. The pH of LiOH was measured to be 11.53. Which of the following statements is/a a. [OH-1 3.4 x 10-3 M b. [H] [OH] c. Solution is basic. d. [H'] 2.95 x 10-12 M

Answer 1

(a) Each conjugate acid-base pair has a characteristic PH range where it works as an effective buffer That range is one PH unit on either side of the pk_a of the conjugate acid PH = Pka = plusminus 1 Pk_a = 9.25: pk_a - 1 = 8.25 7 & p k_a = + 1 = 10.25 Thus PH range is 8.25 to 10.25 Thus the highest PH that a buffer system compared of NH^+_4 will buffer effectively with p k_a = 9.25 is option d) 10.25 A buffer is a solution consisting of equal amount of weak acid and is conjugate base or rice - versa option (c) satisfies the condition After the reaction between NH_3 (aq) and HCl (aq) we are left with NH_3 = 12.5 ml and NH_4 Cl = 12.5 ml & this form buffer solution as NH_3 is as weak base and NH_4 Cl \r\n is it conjugate base