Question

Which of the following solutions would be best to buffer a solution near pH 5? (a) 1.0 x 10‒5 M HCl (b) 1.0 x 10‒5 M NaOH (c) A solution containing approximately equal concentrations of hypochlorous acid (HOCl, Ka = 2.9 x 10‒8) and the dissolved salt of its conjugate base sodium hypochlorite (NaOCl). (d) A solution containing approximately equal concentrations of potassium hydrogen phthalate (C8H5KO4, Ka = 3.91 x 10–6) and the dissolved salt of its conjugate base potassium phthalate. (e) A solution containing approximately equal concentrations of ammonia (Kb = 1.8 x 10‒5) and the dissolved salt of its conjugate acid ammonium chloride.

Answer 1

Option A: 1*10^-5 M HCl has a pH of 5

Step-by-step explanation:

a) 1.0 * 10^‒5 M HCl

HCl = a strong acid

pH of HCl = -log[H+]

ph of a 10^‒5 M HCl = -log(10^-5) = 5

(b) 1.0 * 10^5 M NaOH

pOH = -log(10^-5) = 5

pH = 14 -5 = 9

(c) A solution containing approximately equal concentrations of hypochlorous acid (HOCl, Ka = 2.9 * 10^‒8) and the dissolved salt of its conjugate base sodium hypochlorite (NaOCl).

pH = pKa + log[NaOCl/HOCl]

Since [NaOCl]/ [HOCl]= 1

pH = pKa +log(1)

pH = pKa

pH = -log(2.9 * 10^-8)

pH = 7.54

(d) A solution containing approximately equal concentrations of potassium hydrogen phthalate (C8H5KO4, Ka = 3.91 * 10^–6) and the dissolved salt of its conjugate base potassium phthalate.

pH = pKa +log[potassium phthalate/C8H5KO4)

Since [potassium phthalate]/[C8H5KO4] = 1

pH = pKa

pH = -log(3.91*10^-6)

pH = 5.41

e) A solution containing approximately equal concentrations of ammonia (Kb = 1.8 * 10^‒5) and the dissolved salt of its conjugate acid ammonium chloride.

pOH = pKb +log[NH4Cl]/[NH3]

Since [NH4Cl]/[NH3] = 1

pOH = pKb

pOH = 4.74

pH = 14-4.74 = 9.26