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Properties of Buffer Solutions

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Ajanae Smith

on 20 May 2014

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Transcript of Properties of Buffer Solutions

Properties of Buffer Solutions

Concept
The main objective of this lab is to demonstrate and gain understanding of the following terms:
Important Equations
Dissociation reaction:
HA + H O <==> A + H 0

Ka value:
Ka = 1.7 x 10

Buffer equation:
[H O ] = Ka x [HA]
[A]

pH:
pH = -log [H O ]

Safety Procedures
Please follow all of the safety guidelines as follows:

1. Follow directions carefully
2. DO NOT touch anything until instructed to do so
3. Make sure work space is clean before and after use
Chemical Awareness
Dilute acid and base solutions, including acetic acid, ammonia, citric acid, hydrochloric acid, and sodium hydroxide are skin and eye irritants. Acetic acid and ammonia solutions may be irritating to the respiratory tract
Conclusion
The purpose of this lab was to discover how buffers are made and what properties they withold. In the lab, you conducted an experiment in which you created a buffer at two different bases and where able to view the variations in each solution showing different buffer types and their properties.
Introduction
A buffer protects against rapid changes in pH when acids or bases are added. Every living cell is buffered to maintain constant pH and proper cell function. Consumer products are often buffered to become safe to use in human care.
Be sure to:
1. Wear chemical splash goggles,
2. Chemical-restistant gloves, and a
3. Chemical-resistant apron
pH
Buffer
Weak Acids and Bases
Neutralization
Dissociation constant
The conduction of this lab is also to investigate how buffers are made, the pH range in which they are effective, and their buffer capacity.
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Using a clean graduated cylinder for each solution, measure and add the indicated volumes of the following solutions to five test tubes labeled A-E. Mix the contents of test tubes B,C, and D by gentle stirring, swirling, or shaking
Solution Test Tube
A B C D E
Acetic acid 0.1 M 0 6.7 mL 5 mL 3.3 mL 0
Sodium acetate 0.1 M 0 3.3 mL 5 mL 6.7 mL 0
Distilled water 10 mL 0 0 0 0
pH 7 Buffer 0 0 0 0 10 mL
Avoid exposure of all chemicals to eyes and skin and notify teacher immediately of any spills
Procedures
1. Measure and record the initial pH of the water in test tube A using a pH meter

2. Add 1 drop of 0.1 M HCl to the water in test tube A and measure the pH

If using a pH meter , rinse the electrode with distilled water and blot dry

3. Measure and record the initial pH of the buffer in test tube B

4. Using a graduated pipet or buret, add 1 mL of 0.1 M HCl to the buffer in test tube B. Measure and record the pH

5. Add an additional 2 mL of 0.1 M HCl to test tube B, and again measure the pH

6. Add an additional 3 mL of 0.1 M HCl to test tube B, and once more, measure the pH

Rinse the pH meter with distilled water

7. Repeat steps 3-6 three more times to test the buffer solutions in test tubes C, D, and E

Once done, dispose of the solutions in test tubes A-E. Rinse the test tubes with distilled water and blot dry with a paper towel

8. Refill all test tubes A-E with the designated solutions as provided above

9. Measure the initial pH of the water in test tube A. Add 1 drop of 0.1 M NaOH, and again measure the pH

Clean pH meter as instructed

10. Measure and record the initial pH of buffer B

11. Using a graduated pipet or buret, add 1 mL of 0.1 M NaOH to buffer B. Measure and record the pH

12. Add an additional 2 mL of 0.1 M NaOH to buffer B, and again measure the pH

13. Add an additional 3 mL of 0.1 M NaOH to buffer B, and once more, measure and record pH

If using a pH meter, thoroughly clean as instructed and return to it to its box

14. Repeat steps 10-13 three more times to test the buffer solutions in test tubes C,D, and E

15. Dispose of the solutions in test tubes A-E

Clean up experiment area and make sure all equipment is returned to its proper place, both clean and undamaged
Pre-Lab Answers
1. 1.80
6.60
4.30

2. H PO + H O <-> H PO + H O
HCO H + H O <-> HCO + H O
H BO + H O <-> H BO +H O

3. [H O ] = 1.38 X 10 ; pH=3.86

4. [H O ] = K x [HA]
[A ]

a.) 1.76x10 x (0.67) =3.57 x 10
(0.33)
pH = 4.45
b.)1.76x10 x (0.33) =8.56 x 10
(0.67)
pH =5.06
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How To Use pH meter
1. Remove protective cap

2. Rinse electrode with distilled water and blot dry it with a paper towel prior to use

3. Turn meter on by pressing the ON key

4. Immerse meter into the solution that is to be tested

-Never immerse the electrode up to the connector

5. Stir gently and wait for the reading to stablize

6. Once reading is stabilized, press HOLD button to stop and record reading on meter
-When repeating a measurement, press the HOLD key again and it will automaticallly relieve the hold conditions

7. After use, rinse the electrode with distilled water and blot dry

8. Turn meter off by pressing the OFF key

9. Replace protective cap and place back into box after use



Background Basis
Your Turn!
Using this chart and the explaination I provide you with, you will conduct a short experiment in search of which weak-acid-conjugate pair is the right buffer for you (or at least your assigned challenge)!
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Ammonia is a weak base.
a. Write an equation for the reaction of ammonia with water and identify its conjugate acid
NH + H O ---> NH + OH
NH

b. The pK value for this reaction is 4.7. Recalling that the relationship between pK and pK for a conjugate acid-base pair is pK + pK =14, predict the pH of an equimolar solution containing ammonia and its conjugate acid
pK = 4.7
pK = 14 - 4.7 = 9.3
A buffer solution consisting
of equal moles of NH and NH is
expected to have a pH value of =9.3
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