Question

22) At what pH is hydrofluoric acid 75% dissociated? (K. (HF) = 3.5 x 104) A) 2.98 B) 3.93 C) 3.33 D) 3.58 E) Something else

Answer 1

Sol":- Given : ME Acid ka = 3.58104, as ka is very less , Hence weak Acid. dissociation = 75% : conc. in the not given, let the conc. of HF be c" +=o so HF + + F (aq) (aq) (29) & ff Base present, then Base consumes itt +=o c o o making Reaction to += teq C-0.750 0.750 0.75C move further in 0.25c forward dirh According so, [w*] = 6.750 to Lechatilien's principle Now Py= -log[ut] not given , Hence Py = -log (0.75€) e) something Else , i will well, under acidic cond" (PKT), He will not dissociate to 75% extent as it is a very weak feed Hence, medium has to be basic (PHT) so that dissociation of HF via Base can be increased till 75% go for solni water (neutral) = 152, Powater Salt Added : 18C1 = 450 gms so, o (moles) = 450 (M.) = 53.5 gimo NHAS (molar mass NHC 53.5 of salt) r a, = 8.41 moles O (9) Molarity, Marc = NHC & 8.41 (0.56 mol Soln (int) 15

the Py) via mydrolysis Hence, NH (ad) + WHCL : Strong Aced & weak bore , Hence Acidic salt (HCl) (NHH) "WH (strong congegate Reed) (aq) (aq) (weak conjugate Base, does not (undergoes matte Acidic Hydrolysis) (decreases (No effect on PH) undergo Hydrolysis) Hence, Net caq) + on (04) = NI, OH (29) (1) = 4(04) + oH (29) Adding Rx as we get, dissolution RXN ;- . (09) + 40(d) = NH OH (24) + H+ due to Hydrolysis of NH+ or NHCl(aq) + Hold) – NHOH(aq) + Hellas) Hol is being produced which makes week bare strong send Neutral water Acidic makes system in Off Coo)