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.

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

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