# Advanced Equilibria - Lesson 2 - Calculations Involving Buffers by a.mustafasipsak

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```									                                                              You have seen how buffered solutions
Calculations Involving                              can be made by adding a conjugate salt to a
Buffers                                      weak acid or base, and how the pH of the
buffered solution can be found using the
ICE method or the Henderson-Hasselbalch
equation.
Practice:
Calculate the pH of a buffered solution
containing .700 M methylamine and .500 M
methylaminium chloride.

Two important characteristics of a
buffer include:                                                 2. Buffering Capacity
1. pH Range                                 • The buffering capacity of a buffered
• The pH range is the range of pH values                   solution represents the amount of
over which a buffer system works                         acid or base that can be absorbed by
effectively.                                             the solution without a significant
• The pH range of an acid or base can be                   change in pH.
seen as the region of little pH change vs.
volume of base/acid added on a titration               • A buffer with a large capacity
curve                                                    contains large concentrations of
• It is best to choose an acid with a pKa                  buffering components.
close to the desired pH.

Therefore the capacity of a
1. Calculate the mass of sodium
buffered solution is determined by the
magnitudes of [HA] and [A-].                                 benzoate that must be added to 1 L
of 0.40 M benzoic acid (Ka = 6.4 x
Using the Henderson-Hasselbalch                          10-5) solution to buffer at a pH of
equation, along with these                                   4.5. (Assume no change in total
relationships, we can create a buffer
volume)
system at almost any pH.

[Base]     pOH = pK b + log
[Acid]
pH = pK a + log
[Acid] ,                      [Base]

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When Strong Acids or Bases Are                  In the addition of a strong acid or
Added to a Buffer…                                base to a buffered solution, there
…it is safe to assume that                       are two considerations:
all of the strong acid or
base is consumed in the                       1. How does the added acid-base
reaction.                       react with the acid-base present in
the solution (neutralization)
2. How does the neutralization of
hydronium or hydroxide affect the
equilibrium of the weak acid-base
present in the solution?

Addition of Strong Acid or                                     • Here is a buffered
Base to a Buffer                                            system in which
strong base and
strong acid have
1. Determine how the
neutralization reaction
affects the amounts of the                      • Let us investigate
weak acid and its                                 the results and
conjugate base in solution.                       model the
2. Use the Henderson–                                quantitative
Hasselbalch equation to                           procedures needed
determine the new pH of                           to derive the
the solution.                                     resulting pH’s

Calculating pH Changes in Buffers               Calculating pH Changes in Buffers
Example Problem:                               Before the reaction, since
mol HC2H3O2 and 0.300 mol                         mol HC2H3O2 = mol C2H3O2−
NaC2H3O2 to enough water to
make 1.00 L of solution. The pH
pH = pKa = −log (1.8 × 10−5) = 4.74
of the buffer is 4.74. Calculate the
pH of this solution after 0.020 mol

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Calculating pH Changes in Buffers                       Calculating pH Changes in Buffers
The 0.020 mol NaOH will react with 0.020                Now use the Henderson–Hasselbalch
mol of the acetic acid:                                 equation to calculate the new pH:
HC2H3O2(aq) + OH−(aq) → C2H3O2−(aq) + H2O(l)                         (0.320)
pH = 4.74 + log
(0. 200)
HC2H3O2     C2H3O2−      OH−
pH = 4.74 + 0.06       • This problem could be
solved using an ICE table,
Before reaction     0.300 mol   0.300 mol   0.020 mol
however, it is much simpler
pH = 4.80              to use the Henderson-
After reaction      0.280 mol   0.320 mol   0.000 mol
Hasselbalch equation since
it is a buffered solution.

2. Calculate the change in pH that                      3. Calculate the change in pH that
occurs when 10.00 mL of 0.500 M                         occurs when 0.10 mol of HCL gas is
perchloric acid is added to the buffer                  added to 1 L of a buffered solution
from the example, after the base was                    containing 0.25 M ammonia and 0.40