hydrolysis of salt and ph of buffer solutions

LAB 2

TITLEHydrolysis of salt and pH of buffer solutions.ABSTRACTThis experiment has four objective to be achieved, that are to determine pH values of salts solutions by using different indicators, to calculate the Ka or Kb for each cation or anion that hydrolyzes, preparing an acetic acid-sodium acetate buffer and to investigate the effect of acid on the buffer pH. For Part A, unboiled distilled water is added into each of the six test tubes. Then, three drops of different indicators that are methyl orange, methyl red, bromothymol blue, phenol red, phenolphthalein and alizarin yellow-R is added into each test tubes. The colours changes are then recorded. Approximate pH of these solutions is determined by using indicator chart provided in the laboratory. After that, all the test tubes is emptied and rinsed by using boiled distilled water. All these steps are then repeated by changing unboiled distilled water with boiled distilled water, 0.1 M NaCl, 0.1 M NH4Cl, 0.1 M NaC2H3O2, 0.1 M ZnCl2, 0.1 M KAI(SO4)2, and 0.1 M Na2CO3. Then, Part B is the preparation of acetic acid-sodium acetate buffer and to determine the effect of acid and base on buffer pH. The pH of buffer solution prepared is 5.598 but after the addition of acid, the pH decreases to 5.141 and after the addition of base, the pH increases to 6.698.OBJECTIVES1. To determine pH values of salts solutions by using different indicators.2. To calculate the Ka or Kb for each cation or anion that hydrolyzes.3. To prepare acetic acid-sodium acetate buffer.4. To investigate the effect of acid and base on the buffer pH.APPARATUS500 mL Erlenmeyer flask, 150 mL beaker, 10 mL measuring cylinder, 100 mL measuring cylinder, 6 sets of test tubes, test tubes rack, stirring rod, pH meter, analytical balance, pipette, hot plate, plastic water bottle

CHEMICALSNaC2H3O2.3H2O, 0.1 M ZaCl2, 0.1 M NH4Cl, 0.1 M KAl (SO4)2, 0.1 M Na2CO3, 0.1 M NaCl, 0.1 M NaC2H3O2, 3.0 M HCl, 3.0 M HC2H3O2, 3.0 M NaOHDropper bottles of : Methyl orange, Methyl red, Bromothymol blue, Phenolphthalein, Alizarin yellow-R, Phenol red, Standard buffer solution (pH 4.5)PROCEDUREInitial step (prepare by lab technician)Boil 1L distilled water and allow it to cool to room temperature before starting the experiment.Part A : pH solution and hydrolysis of salts1. 5mL of unboiled distilled water is added to each of six test tubes.2. Three drops of different indicators is added into each six test tubes. The indicators to use are: methyl orange, methyl red, bromothymol blue, phenol red, phenolphthalein, and alizarin yellow-R.3. The colours obtained are recorded.4. The approximate pH of unboiled distilled water samples to the nearest pH is determined by using provided indicator chart.5. The test tubes is emptied and is rinsed three times with 3mL boiled distilled water.6. Step 1 until step 5 is repeated by changing unboiled distilled water with boiled distilled water, 0.1 M NaCl, 0.1 M NH4Cl, 0.1 M NaC2H3O2, 0.1 M ZnCl2, 0.1 M KAl(SO4)2, and 0.1 M Na2CO3.Part B : pH of buffer solutioni. Preparation of acetic acid-sodium acetate buffer

1. 3.50 g of NaC2H3O2.3H2O is weighed.2. The mass is recorded and is put into a 150 mL beaker. The beaker is labelled as beaker 1.3. 8.8 mL of 3 M acetic acid is measured and is added to beaker 1.4. 55.6 mL of distilled water is then measured and also added to beaker 1.5. The solution is stirred until all the sodium acetate is dissolved.6. The pH of the solutions is measured using calibrated pH meter.7. The solutions are saved for Part (ii).

ii. Effect of acid and base on buffer pH

1. 32 mL of prepared buffer solution is poured into two 150 mL beakers. The beakers are then labelled as beaker 2 and beaker 3.2. 1 mL of 3.0 M HCl is pipette into beaker 2 and the pH of the resultant solution is measured.3. 1 mL of 3.0 M NaOH is pipette into beaker 3 and the pH of the resultant solution is then measured.

RESULTS AND DISCUSSIONSPart A : pH solution and hydrolysis of saltsSolution: unboiled distilled waterINDICATORMethyl OrangeMethyl RedBromothymol BluePhenol RedPhenolphthaleinAlizarin Yellow-R

pH4.05.42.07.08.010.0

COLOUR CHANGESOrangePalePinkPaleYellowYellowish OrangeColourlessYellow

Solution: boiled distilled waterINDICATORMethyl OrangeMethyl RedBromothymol BluePhenol RedPhenolphthaleinAlizarin Yellow-R

pH4.05.43.07.77.010.0

COLOUR CHANGESOrangeLight PinkVery Light YellowYellowColourlessYellow

Solution: 0.1 M NaClINDICATORMethyl OrangeMethyl RedBromothymol BluePhenol RedPhenolphthaleinAlizarin Yellow-R

pH4.05.41.07.07.09.0

COLOUR CHANGESOrangePalePinkVery Light YellowLight OrangeColourlessLight Yellow

Solution: 0.1 M NH4ClINDICATORMethyl OrangeMethyl RedBromothymol BluePhenol RedPhenolphthaleinAlizarin Yellow-R

pH4.05.82.07.07.010.0

COLOUR CHANGESOrangePinkVery Light YellowYellowColourlessYellow

Solution: 0.1 M NaC2H3O2INDICATORMethyl OrangeMethyl RedBromothymol BluePhenol RedPhenolphthaleinAlizarin Yellow-R

pH5.06.57.38.48.011.0

COLOUR CHANGESYellowLight YellowLight BluePinkColourlessOrange

Solution: 0.1 M ZnCl2INDICATORMethyl OrangeMethyl RedBromothymol BluePhenol RedPhenolphthaleinAlizarin Yellow-R

pH4.35.85.87.08.010.3

COLOUR CHANGESPale OrangePale YellowPale Green OrangePale PinkSlightly Orange

Solution: 0.1 M KAl(SO4)2INDICATORMethyl OrangeMethyl RedBromothymol BluePhenol RedPhenolphthaleinAlizarin Yellow-R

pH3.65.54.56.08.010.0

COLOUR CHANGESBrick-RedVery Light PinkVery Light YellowLight YellowColourlessLight Yellow

Solution: 0.1 M Na2CO3INDICATORMethyl OrangeMethyl RedBromothymol BluePhenol RedPhenolphthaleinAlizarin Yellow-R

pH4.06.87.38.09.311.0

COLOUR CHANGESOrangeYellow-OrangeBluePink Dark PinkReddish Orange

Part B : pH of buffer solution(i) Preparation of acetic acid-sodium acetate bufferWeight of NaC2H3O2.3H2O3.5014 g

Volume of 3.0 M acetic acid8.8 mL

Volume of distilled water55.6 mL

pH of the buffer solution5.598

(ii) Effect of acid and base on buffer pHpH of the solutions in beaker 15.598

pH of the solutions in beaker 2 (addition of 1mL of 3.0 M HCl)5.141

pH of the solutions in beaker 3 (addition of 1mL of 3.0 M NaOH)6.698

Formula:

KW = [H+] [OH-] = 1.0 x 10-14pH = -log [H+][OH-] = Average pH = Kw = Ka KbKa = Kb =

CALCULATIONSUnboiled distilled waterAverage pHAverage pH = = = 6.07

Concentration of [H+]pH = -log [H+]6.07 = -log [H+][H+] = 8.51 x 10-7

Concentration of [OH-][OH-] = = = 1.18 x 10-8

Net ionic equationH2O (l) H+ (aq) + OH-(aq)CO2 (aq) + H2O (aq) H2CO3 (aq)H2CO3 (aq) H+ (aq) + HCO3- (aq)

KaKa = [HCO3- ] [H+ ] = 4.2 x 10-7 [H2CO3]

KbKb = 1.0 x 10-14 = 2.38 x 10-81.2 x 10-7

Boiled distilled waterAverage pHAverage pH = = =6.18



Net ionic equationH2O (l) H+ (aq) + OH-(aq)

KaKa = [H+ ] [OH- ] = (6.61 x 10-7) (1.52 x 10-8 ) = 1.0 x 10-14 = Kw

KbKb = 1.0 x 10-14 = 1.00 x 1001.0 x 10-14

0.1M NaClAverage pHAverage pH = = =5.57



Net ionic equationNaCl (l) Na+ (aq) + Cl-(aq)H2O (l) H+ (aq) + OH-(aq)CO2 (aq) + H2O (aq) H2CO3 (aq)H2CO3 (aq) H+ (aq) + HCO3- (aq)

Ka-

Kb-

0.1M NH4ClAverage pHAverage pH = = =5.97



Net ionic equationNH4Cl (l) NH4+ (aq) + Cl- (aq)NH4+ (aq) + H2O (l) NH3(aq) + H3O+ (aq)

KaKa = [NH3][H+] = 5.6 x 10-10 [NH4+]

KbKb = 1.0 x 10-14 = 1.79 x 10-5 5.6x 10-10

0.1 M NaC2H3O2Average pHAverage pH = = = 7.7


Concentration of [OH-][OH-] = = = 5 x 10-7

Net ionic equationNaC2H3O2 (aq) Na+(aq) + C2H3O2- (aq)C2H3O2- (aq) + H2O (l) C2H4O2 (aq) + OH-(aq)

KaKa = 1.0 x 10-14 = 4.0 x 104 2.5 x 10-19

KbKb = [C2H4O2] [OH-] = 2.5 x 10-19 [C2H3O2

0.1 M ZnCl2Average pHAverage pH = = =6.87



Net ionic equationZn2+ (aq) + H2O (l) Zn(OH)2 (aq) + H+ (aq)

KaKa = [Zn(OH)2] [H+] = 2.5 x 10-7 [Zn2+]

KbKb = 1.0 x 10-14 = 4.0 x 10-8 2.5 x 10-7

0.1 M KAl(SO4)2Average pHAverage pH = = =6.23



Net ionic equationKAI4+ (aq) + H2O (l) KAI(OH)4 (aq) + H+(aq)

KaKa = [KAI(OH)4] [H+ ] = 6.3 x 10-7 [KAI4+]

KbKb = 1.0 x 10-14 = 1.59 x 10-8 6.3 x 10-7

0.1 M Na2CO3Average pHAverage pH = = =7.73



Net ionic equationNa2CO3 (l) 2Na+ (aq) + CO32- (aq)CO32- (aq) + H2O (l) HCO3- (aq) + OH- (aq)

KaKa = 1.0 x 10-14 = 4.76 x 10-11 2.1 x 10-4

KbKb = [HCO3- ] [OH- ] = 2.1 x 10-4 [CO32-]

DICUSSIONFor the part A, the experiment is done by adding 5ml of unboiled water to six test tubes.Three drops of six different indicator (methyl orange, methyl red, bromothymol blue, phenol red, phenolphthalein and alizarin yellow-R) is then added into each of the test tubes.From the changes of colour of the indicator, the pH value can be determine by referring the colour change to the indicator chart.The experiment is repeated by changing the unboiled water with boiled water, 0.1M NaCl, 0.1M NH4Cl, 0.1M NaC2H3O2, 0.1M ZnCl2, 0.1M KA(SO4)2, and Na2CO3.For unboiled distilled water, the pH that we get is 6.07. The concentration of H+ and OH- is 8.51 x 10-7 and 1.18 x 10-8.The net equation for this hydrolysis is,H2O (l) H+ (aq) + OH- (aq)CO2 (aq) + H2O (aq) H2CO3 (aq)H2CO3 (aq) H+ (aq) + HCO3- (aq)The expression for equilibrium constant is,Ka = [HCO3-] [H+] [H2CO3]

And the value of Ka and Kb that we get is 4.2 x 10-7 and 2.38 x 10-8.

For boiled distilled water, the pH that we get is 6.18. The concentration of H+ and OH- is 6.61 x 10-7 and 1.52 x 10-8The net equation for this hydrolysis is,H2O (l) H+ (aq) + OH-(aq)The expression for equilibrium constant is,Ka = [H+ ] [OH- ] And the value of Ka and Kb that we get is 1.0 x 10-14 and 1.00 x 100.For 0.1 M NaCl, the pH that we get is 5.57. The concentration of H+ and OH- is 2.69 x 10-6 and 3.72 x 10-9The net equation for this hydrolysis is,H2O (l) H+ (aq) + OH-(aq)CO2 (aq) + H2O (aq) H2CO3 (aq)H2CO3 (aq) H+ (aq) + HCO3- (aq)There is no Ka or Kb for this hydrolysis.

For 0.1 M NH4Cl, the pH that we get is 5.97. The concentration of H+ and OH- is 1.07 x 10-6 and 9.34 x 10-9The net equation for this hydrolysis is,NH4Cl (l) NH4+ (aq) + Cl- (aq)NH4+ (aq) + H2O (l) NH3(aq) + H3O+ (aq)The expression for equilibrium constant is,Ka = [NH3] [H+] [NH4+]

And the value of Ka and Kb that we get is 5.6 x 10-10 and 1.79 x 10-5.

For 0.1 M Na2C2H3O2, the pH that we get is 7.7. The concentration of H+ and OH- is 2 x 10-8 and 5x 10-7.The net equation for this hydrolysis is,NaC2H3O2 (aq) Na+(aq) + C2H3O2- (aq)C2H3O2- (aq) + H2O (l) C2H4O2 (aq) + OH-(aq)The expression for equilibrium constant is,Kb = [C2H4O2] [OH-] [C2H3O2-]And the value of Ka and Kb that we get is 4.0 x 104 and 2.5 x 10-19

For 0.1 M ZnCl2, the pH that we get is 6.87. The concentration of H+ and OH- is 1.35 x 10-7 and 7.41 x 10-8The net equation for this hydrolysis is,Zn2+ (aq) + H2O (l) Zn(OH)2 (aq) + H+ (aq)The expression for equilibrium constant is,Ka = [Zn(OH)2] [H+] [Zn2+]And the value of Ka and Kb that we get is 2.5 x 10-7 and 4.0 x 10-8.

For 0.1 M KAI (SO4)2, the pH that we get is 6.23. The concentration of H+ and OH- is 5.88 x 10-7 and 1.7 x 10-8.The net equation for this hydrolysis is,KAI4+ (aq) + H2O (l) KAI(OH)4 (aq) + H+(aq)The expression for equilibrium constant is,Ka = [KAI(OH)4] [H+] [KAI4+]And the value of Ka and Kb that we get is 6.3 x 10-7 and 1.59 x 10-8

For 0.1 M Na2CO3, the pH that we get is 7.73. The concentration of H+ and OH- is 1.86 x 10-8 and 5.38 x 10-7.The net equation for this hydrolysis is,Na2CO3 (l) 2Na+ (aq) + CO32- (aq)CO32- (aq) + H2O (l) HCO3- (aq) + OH- (aq)The expression for equilibrium constant is,Kb = [HCO3- ] [OH- ] [CO32-]And the value of Ka and Kb that we get is 4.76 x 10-11 and 2.1 x 10-4.

For experiment in part B, acetic acid is added to the NaC2H3O2.3H2O .The pH that we get after measuring the pH of the solution using a calibrated pH meter is 5.598.

After the addition of 3.0 M HCl, the pH increase to 5.141. This pH value can be said valid because the number of pH is decrease as the solution is becoming more acidic.However. after the addition of 3.0 M NaOH, the pH is increasing to 6.698. This pH value is valid because the number of pH is increase as the solution becomes more alkaline.

CONCLUSIONFrom the experiment, we get the pH value of unboiled distilled water 6.07, boiled distilled water 6.18, 0.1 M NaCl 5.57, 0.1 M NH4Cl 5.94, 0.1 M NaC2H3O2 7.7, 0.1 M ZnCl2 6.87, 0.1 M KAI (SO4)2 6.23, 0.1 M Na2CO3 7.73.For salts derived from a weak acid and a weak base, both the cation and anion hydrolyze. However, whether a solution containing such a salt is acidic, basic or neutral depends on the relative strengths of the weak acid and weak base. If Ka is more than Kb, then the solution is acidic solution and vice versa.

A buffer solution resists large changes in pH upon the addition of small amounts of strong acid or strong base. The pH value will decrease when buffer solution react with acid and will increase when react with base. But in this experiment, some errors had occurred that make the pH value after the addition of acid increase to 4.948 and not decrease.

All the objectives of this experiment which are to determine pH values of salts solutions by using different indicators, to calculate the Ka or Kb for each cation or anion that hydrolyzes, preparing acetic acid-sodium acetate buffer and to investigate the effect of acid on the buffer pH has been achieved.

REFERENCES

Chang R. (2008). Chemistry (9th Ed) New York, McGraw-Hill Companies, Inc. (Page 660-607) Carnegie Mellon University (2001), Handout On Buffer Solutions, 30 July 2010 from http://www.chem.cmu.edu/courses/09-106/notes/buffers.pdf Jim clark (2002), Buffer Solutions, 31 July 2010 from http://www.chemguide.co.uk/physical/acidbaseeqia/buffers.html Dhanlal De Lloyd (2000), Preparation of pH Buffer Solutions, 31 July 2010 from http://delloyd.50megs.com/moreinfo/buffers2.html Brad Cole (2010), What Is Buffer Solutions?, 28 July 2010 from http://www.wisegeek.com/what-is-a-buffer-solution.htm

hydrolysis of salt and ph of buffer solutions

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