A weak acid HA has an acid dissociation constant K_a = 1.0 times 10^-5. For a 0.
ID: 522986 • Letter: A
Question
A weak acid HA has an acid dissociation constant K_a = 1.0 times 10^-5. For a 0.30 M solution of NaA in water, which of the following is true? A. [HA] > [A^-] B. [H^+] > [A^-] C. [H^+] > [OH^-] D. [H^+] = (K_a[A^-]^1/2 (K_w/K_a)^1/2 If solid (CH_3)_2NH_2Cl is dissolved in an aqueous solution of (CH_3)_2NH with an initial pH of 8.13, which of the following statements will be true? A. The K_b for the base hydrolysis of (CH_3)_2NH will decrease. B. The pH of the solution will decrease. C. The hydroxide ion concentration will increase. D. Chlorine gas will be produced. E. The free energy change Delta G for the base hydrolysis of (CH_3)_2 NH will decrease.Explanation / Answer
NaA+ H2O ------->HA+OH-
Kb= [HA] [OH-]/ [A-] = KW/Ka since [HA] = [OH-] = x
Hence x2= [A-] KW/Ka, [OH-]2 = [A-] (KW/Ka)
[OH-] = sqrt{ [A-] Kw/Ka}, E is correct.
2.
CH3)2NH2Cl -------à(CH3)2NH2+CL-
(CH3)2NH2+ H2O ------à(CH3)2NH+H3O+
So hydronium ions are produced when (CH3)2NH2 is dissolved in water. The reaction increases the concentration of hydronium ions, which in turn will make the solution acidic.
This reduces the pH of resulting solution ( B is correct)
2.