known: pk, 2 PROCEDURE Safety Notes: Use appropriate safety precautions for work
ID: 495853 • Letter: K
Question
known: pk, 2 PROCEDURE Safety Notes: Use appropriate safety precautions for working with concentrated acids and bases. This includes splash-proof goggles, ong pants, and closed-toe shoes. No eating, drinking, or chewing at any time in the laboratory. PART I: DETERMINATION OF K AND KA obtain a pH meter and calibrate if necessary Collect a 400-mL beaker and fi with distilled water to -200 mL. You m L w 200 M Collect a hot plate and set the water on the plate to boil. Boil the 200 Lof Boit cool mL distilled water for about 5 minutes Allow the water to cool to room temperature. while you are waiting for the water to boil and cool, obtain the about 25 mL of distilled water in a 50-mL beaker. Measure and record the pH ofthe boiled and the unboiled water. Using large test tubes, obtain enough of each of the six salt solutions provided to cover the probe tip of your pH meter. Measure and record the ph of each solution. Be sure to rinse the tip of the probe with boiled distilled water and pat dry between each solution. EXERCISE 8 I BUFFERS AND SALT HYDROLYSIS 116Explanation / Answer
Data Table 1
Solutions Hydrolyzing ion Spectator ion
0.1 M NaBr H+, OH- Na+, Br-
0.1 M Na2CO3 CO3^2- Na+
0.1 M NaOAc AcO- Na+
0.1 M NH4Cl NH4+ Cl-
0.1 M ZnCl2 Zn2+ Cl-
0.1 M KAl(SO4)2 Al(SO4)2- K+
Data Table 3 : Ka and Kb calculations
Solutions net-ionic equation for hydrolysis Ka or Kb expression
Na2CO3 CO3^2- + H2O <==> HCO3- + OH- Kb = [HCO3-][OH-]/[CO3^2-]
NaOAc AcO- + H2O <==> HOAc + OH- Kb = [HOAc][OH-]/[AcO-]
NH4Cl NH4+ + H2O <==> NH3 + H3O+ Ka = [NH3][H3O+]/[NH4+]
ZnCl2 Zn2+ + 2H2O <==> Zn(OH)2 + 2H+ Kb = [Zn(OH)2][H+]^2/[Zn2+]
KAl(SO4)2 Al(SO4)2- + H2O <==> HAl(SO4)2 + OH- Kb = [HAl(SO4)2][OH-]/[KAl(SO4)2]
pH of buffer solutions
pH of original buffer
[HOAc] = 8.8 g/60.05 g/mol x 0.0556 L = 2.63 M
[AcO-] = 3.5 g/82.03 g/mol x 0.0556 L = 0.77 M
pH = pKa + log(base/acid)
= 4.74 + log(0.77/2.63) = 4.21
pH of buffer solution after adding HCl
[HCl] before reaction = 6 M x 1 ml/33.2 ml = 0.181 M
[HOAc] before reaction = 2.63 M x 32.2 ml/33.2 ml = 2.55 M
[AcO-] before reaction = 0.77 M x 32.2 ml/33.2 ml = 0.75 M
[HOAc] after reaction = 2.731 M
[AcO-] after reaction = 0.569 M
pH = 4.74 + log(0.569/2.731) = 4.06
pH of buffer solution after adding NaOH
[NaOH] before reaction = 6 M x 1 ml/33.2 ml = 0.181 M
[HOAc] before reaction = 2.63 M x 32.2 ml/33.2 ml = 2.55 M
[AcO-] before reaction = 0.77 M x 32.2 ml/33.2 ml = 0.75 M
[HOAc] after reaction = 2.369 M
[AcO-] after reaction = 0.931 M
pH = 4.74 + log(0.931/2.369) = 4.33
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