Question

Data Tables:

Drops of   HCl added	pH Beaker 1 Acetic Acid + Acetate	pH Beaker 2 Ammonia + Ammonium	pH Beaker 3 Water
0 drops	4.86	9.55	5.49
2drops	4.76	9.46	2.61
4drops	4.61	9.35	2.34
6 drops	4.50	9.24	2.17
8 drops	4.36	9.12	2.06
10drops	4.21	8.96	1.99
12drops	4.02	8.75	1.90

Drops of NaOH added	pH Beaker 1 Acetic Acid + Acetate	pH Beaker 2 Ammonia+ Ammonium	pH Beaker 3 Water
0 drops	4.97	9.72	8.90
2drops	5.04	9.76	11.68
4drops	5.10	9.82	12.00
6drops	5.17	9.88	12.20
8drops	5.24	9.95	12.33
10drops	5.32	10.02	12.40
12drops	5.40	10.10	12.50

Post lab Questions (written or type out on a separate sheet):

(Q1) According to your graphed data is one buffer (beaker 1-3) better or worse than the other(s)? Explain.

(Q2) Write out the chemical reaction that occurs in the Acetic Acid / Acetate buffer when

NaOH is added

HCl is added

(Q3) Write out the chemical reaction that occurs in the Ammonia/Ammonium buffer when

NaOH is added

HCl is added

(Q4) Write out the chemical reaction that occurs in the Distilled water when

NaOH is added

HCl is added

(Q5) Does distilled water make a good buffer? Why or why not?

(Q6) How can you tell if the buffer is working?

(Q7) What happens when you go beyond the buffer capacity? How is buffer capacity indicated on your graph(s)?

Answer 1

Q1.

The simplest definition of a buffer is a solution that resists changes in hydrogen ion concentration as a result of internal and environmental factors. Buffers essentially maintain pH for a system.

Here,

Change in pH on adding 12 drops of HCl,

Buffer-1 = 4.86 - 4.02 = 0.82

Buffer-2 = 9.55 -8.75 = 0.80

Buffer -3 = 5.49 - 1.90 = 3.59

Change in pH on adding 12 drops of NaOH,

Buffer-1 = 5.40 - 4.97 = 0.43

Buffer-2 = 10.10 -9.72 = 0. 38

Buffer -3 = 12.50 -8.90 = 3.60

As, you can see the change in pH is less in Buffer-2 as compared to other buffers. Hence, buffer-2 is the best buffer. The increasing order is : Buffer-3 < Buffer-1 <Buffer-2.

Q2

With NaOH :

CH3COOH + OH- --> CH3COO- + H20

With HCl

CH3COOH + H2O -----> H3O+ + CH3COO-

Q3.

With NaOH :

CH3COOH + OH- --> CH3COO- + H20

NH3+NaOH=NH3+H2O+Na

With HCl

NH3(aq)+HCl(aq)=NH4CI(aq)

Q4.

With NaOH :

NaOH + H2O---> Na+ + OH- +H20

With HCl

H2O + HCL ---> [H3O+] + Cl-

Q5.

No, distilled water can not make a good buffer as it has no resistance to change in ph whicch is a defining characteristic of a buffer, buffers are typically composed of a weak acid and it's conjugate base or of a weak base and its conjugate acid.

Q6.

For certain experiments, however, it is desirable to keep a fairly constant pH while acids or bases are added to the solution either by reaction or by the experimenter. Buffers are designed to fill that role. Chemists use buffers routinely to moderate the pH of a reaction. Biology finds manifold uses for buffers which range from controlling blood pH to ensuring that urine does not reach painfully acidic levels. So , if the change in pH is very low even after adding a large amount of base or acid than we can say that the buffer is working.

Q7.

The buffering capacity of a solution refers to the amount of acid or base that can be added to the solution without going beyond the buffering range. When you go beyond the buffer capacity , the buffer slowly starts acting as normal solvent and the change in pH starts increasing/decreasing drastically.

Buffer capacity definition that takes this intuition into account is given by

where n is number of equivalents of added strong base (per 1 L of the solution)