Question

1 ) Using the data in the table below, which conjugate acid is the weakest acid, given that all solutions (aq., 25 °C) are 0.200 M? Explain your choice! Base Kb CIO 3.3 x 10-7 CO3-2 1.8 x 10-4 HS- 1.8 x 10-7 NH2CH3 4.4 x 10-4 1b) Which of the following aqueous solutions has the lowest [OH-] (aq., 25 °C)? A) a solution with a pH of 3.0 B) a 1 x 10-4 M solution of HNO3 C) a solution with a pOH of 12.0 D) pure water E) a 1 x 10-3 M solution of NH4Cl

Answer 1

#1. Kb(base dissociation constant) value indicates the strength of the base .

More will be the strength of base , more will be its Kb value and a strongest base will have weakest conjugate acid .

Thus , NH2CH3 having highest Kb value is strongest base out of all given options and its conjugate acid will be the weakest acid .

#1(b).

(A) pH + pOH = 14 , then pOH = 14 - pH

pOH for solution having pH 3 is = 14-3 = 11

[OH^-] = 10^-pOH = 1×10^-11 M

(B) HNO3 is a strong acid and it completely dissociates into H^+(aq) and NO3^-(aq) .

[H^+] = concentration of HNO3 = 1×10^-4M

[H^+] [OH^-] = 1×10^-14

Therefore, [OH^-] = 1×10^-14 /[H^+] = 1×10^-14 / 1×10^-4 = 1×10^-10 M

(C) [OH^-] = 10^-pOH = 1×10^-12 M

(D) pure water is neutral having pH = pOH = 7

Therefore [OH^-] = 1×10^-7M

(E) Ka for NH3 = 5.6×10^-10

[H^+] = (Ka × concentration)^1/2

[H^+] = (5.6×10^-10 × 1×10^-3)^1/2

[H^+] = 1.8×10^-6M

[OH^-] = 1×10^-14 /1.8×10^-6 = 5.6×10^-9M

Thus option (C) a solution with pOH 12 have lowest [OH^-] i.e. 1×10^-12M