What concentration of ammonia, [NH_3], should be present with [NH^+_4] = 0.732 M

Question

What concentration of ammonia, [NH_3], should be present with [NH^+_4] = 0.732 M to produce a buffer solution with pH = 9.12? For NH_3, K_b = 1.8 times 10^-5 A solution of volume 75.0 mL contains 15.5 mmol HCOOH and 8.50 mmol NaHCOO. (a) What is the pH of this solution? (b) If 0.25 mmol Ba (OH)_2 is added to the solution, what will be the pH? (c) If 1.05 mL of 12 M HCl is added to the original solution, what will be the pH? This single equilibrium equation applies to different phenomena described in this or the preceding chapter. CH_3COOH + H_2O H_3O^+ + CH_3COO^- Of these four phenomena, ionization of pure acid, common-ion effect, buffer solution, and hydrolysis, indicate which occurs if (a) [H_3O^+] and [CH_3COOH] are high, but [CH_3COO^-] is very low. (b) [CH_3COO^-] is high, but [CH_3COOH] and [H_3O^+] are very low. (c) [CH_3COOH] is high, but [H_3O^+] and [CH_3COO^-] are low. (d) [CH_3COOH] and [CH_3COO^-] are high, but [H_3O^+] is low. Write solubility equilibrium equations that are described by the following K_sp expressions. For example, K_sp = [Ag^+][Cl^-] represents AgCl(s) Ag^+(aq) + Cl^-(aq) (a) K_sp = [Fe^3+][OH^-]^3 (b) K_sp = [BiO^+][OH^-] (c) K_sp = [Hg^2+_2][I^-]^2 (d). K_sp = [Pb^2+]^3[AsO^3-__4]^2

Explanation / Answer

Question 12.

Find [NH3]

for

[NH4+´] = 0.732 M to have a pH= 9.12

so

Kb = 1.8*10^-5

pKb = -log(Kb) = -log(1.8*10^-5) = 4.75

apply Buffer equation

pOH = pKb + log(NH4+/NH3)

pOH = 14-pH = 14-9.12 = 4.88

so

4.88 = 4.75 + log(0.732/NH3)

[NH3] = 1/0.732 * 10^-(4.88 -4.75) = 1.0127 M of NH3 required

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