Question

Part A Arsenic acid (H3ASO4) is a polyprotic acid with Ka1 = 5.5x10-3, Ka2 = 1.7x10-7, and Ka3 = 5.1*10-12 Which statement describes best the pH of a 0.010 M solution of this acid. (Hint: Start with calculating the pH as if this were a monoprotic acid.) O The value of pH is somewhat higher than 2.13. O The value of pH is somewhat lower than 2.13. O The value of pH is somewhat higher than 6.65. The value of pH is 2.13. O The value of pH is 6.65.

Answer 1

part A ( First friston dissociation) . H₃ A8O4 - Hot Ho ABO initially 0.01M at eam 0.01 (-x) 0.014 0.012 x= degece of dissociation Thus, . THA] [H2 ASO4 ] | Ka, = [H3 Ao 04] > Ka, = . Que Volu x 0.01% .:: 0,0111-x) since (1-2) n 1 c weak aud) 0.01 - Ka, = 0.0ixe? = x - Kas 0.01 15.5x103 V55 x102 7.42×101 :.= 0.742

Thies, [HF] = 0.01 i = 0.01 X 0.742 i = 7.42 x103, ..pH = -log[ht] = -log(7.42) - log (10) BATT METTE A = -0.87+3 = 2.13 Que to second & third proton dissociation HzAs Of = H + Had0f2 HASO Z HO + Ason The concentration of Ht increases (even by small amount). So, pH < 2013 : Thus, corrent option: 6 • The value of pH is somewhat lower than 2013