Chem 201 Lab Report 3 PDF

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Chem 201-12 2/12/2018 Emily Tafoya Lab Partner: Tammie Chem 201, Experiment 3 Using pH Meter to Measure Ka and Kb Procedure Refer to the pre-lab report on page 10 of my lab notebook. Experimental Data: Sample NH3 NH4+

pH of 0.1M 11.12 5.58

pH of 0.001M 10.17 6.11

Calculated Results: NH3 Concentration

pOH

[H3O+]

[OH-]

[NH4+]

Kb

% [NH4+]

0.1 M

2.88

x10-12M

1.3x10-3M

1.3x10-3M

1.7x10-5

1.3%

.001 M

3.83

x10-11M

1.5x10-4M

1.5x10-4M

2.2x10-5

14.8%

NH4+ Concentration

pOH

[H3O+]

[OH-]

[NH3]

Ka

% [NH3]

0.1 M

8.42

2.6x10-6M

x10-9M

x10-3M

2.6x10-6

0.0026%

.001 M

7.89

7.8x10-7M

x10-8M

x10-4M

5.24 x 10-10

0.072%

Acid Dissociation Rxn: NH4 (aq) + H2O = NH3-(aq) + H3O+(aq) +

Initial Change Equilibrium

NH4+ .1M -x .1-x

NH30 +x x

H3O+ 0 +x x

Initial Change Equilibrium

NH4+ .001 -x .001-x

NH30 +x x

H3O+ 0 +x x

Initial Change Equilibrium

[NH3-] .1 -x .1-x

[NH4+] 0 +x x

[OH-] 0 +x x

Initial Change Equilibrium

[NH3-] .001 -x .001-x

[NH4+] 0 +x x

[OH-] 0 +x x

Ka= [NH3--][H3O+]/[NH4+] Base Dissociation Rxn: NH3-(aq) +H2O = NH4+(aq) + OH-(aq)

Kb = [NH4+] [OH-] / [NH3-]

Discussion: In this experiment my lab partner and I determined both the acid dissociation constant, Ka, of NH4+, and the base dissociation constant, Kb, of NH3- in two solutions of with the concentrations of 0.1 and 00.1 molarity. NH4+ is a weak acid, so the Ka values were reasonable at 2.6x10-6 and 7.8x10-7 as they both came from pH values less than seven, 5.58 and 6.11. NH3- is a weak base, so the Kb values were reasonable at 1.7x10-5 and 2.2x10-5 as they both came from pH values greater than 7, 11.12 and 10.17. Sources of error within the experiment could have come from insufficient cleaning of the pH meter with a kimwipe and DI water, insufficient reading and calibration of the pH meter, random error in the meter’s readings, not making sure the pH meter was stable, or inaccurate dilution with the pipettes used to deliver the solutions. The pH values measure for the acids were both below a pH of seven, and the pH values measured for the bases were both above a pH of seven, meeting my expectations as all values were still close to seven making them weak acids and bases. The dilutions of the concentrations of both the weak acids and weak bases caused the pH’s of both to move closer to seven. When the concentration is decreased, Ka decreases as well. This should happen as the acid doesn’t dissociate as much as there is less acid in the solution. When you decrease the concentration the % of the compound increases as a larger Ka leads to a larger % ionization....