Question

Consider the following data on some weak acids and weak bases: acid name formula name hydrocyanic acid HCN 4.9 x 100 base formula hydroxylamine HONH 1.1*10* methylamine CH, NH 4.4 x 10 hydrofluoric acid HF 6.8 x 10 Use this data to rank the following solutions in order of increasing pH. In other words, select a 'l' next to the solution that will have the lowest pH, a 2' next to the solution that will have the next lowest pH, and so on. solution 0.1 M NACI choose one 0.1 M NON choose one 0.1 MKF 0.1 MCH NHC choose one: Explanation Check Remote

Answer 1

(9) 0.1 m Nach 2 It is the salt of st7099 acid (HCl) and strong base (HaOH). Therefore it does not undergo hydrolysis. NaCl + H2O - NaOH + HCl ... pH is neutral ie. [7] (b) on M Nach Salt of weak acid (HCN) and strong base ( Ngon). 1 and pka of HCM is 4.9x10 to . . pka = logika = -log (4.9 x10 ") = 9.31 - and pH = { pkw + 1 pka - I loge = {x14 + +*431 - log on .7+ 4.65 - I(-1) - 7+ 4.65+0.5 pH = 12.15

(C) 0.1 M KF Salt of weak acld (HF) and strong base ( trot). and ka of HF is 6.8 x104. pka = -logika E-log (68x10 ") = 3.17 and pH = 1 pkw + ₃ pka - I loge • £x14 + 2x3.12. - £ log 0.11 - 7+ 1.585 +0.5 pH = 9.08 (d) 0.7 M CH3 NHz C1 Salt of strong acid (HCI) and wealt base (cus NH2) and kb of CH3NH2 is 4.4x104. A. pks = -log ko = -log(4.4x10*) = 3.36

and pH = £phew - 2 pkb + I loge + 4x14 - *3.36 +5log of - 7 - 1.68 -0.5 pH = 4.82 Therefore the decreasing order of pH of the solution is- 0.1m Nach > 0.1 m KF > 0.1 m Nace > o.1 M CH3 NAC" (I) (11 (111) (1)