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Wize University Chemistry Textbook > Buffers and Titrations

Foming Buffers

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Forming Buffers

Wize Tip
Exams commonly ask questions about which combinations of molecules could form buffers!

To determine if compounds will form a buffer we need to look for a few things:

Do we have a weak acid and its conjugate base? If we do, this will form a buffer!

If we have a strong acid/base reacting with a weak acid/base, we might form a buffer:

  1. If we react an equal amount of strong acid/base with weak base/acid then there will be a chemical reaction but no buffer since there is no weak substance left Example:
HCl  +  NH3 → Cl− + NH4+HCl\ \ +\ \ NH_3\ \rightarrow\ Cl^-\ +\ NH_4^+


  1. If we react a strong acid/base in excess with a weak base/acid and of the strong substance then there will be a chemical reaction but no buffer because there is excess of the strong substance and no weak substance left Example:
HCl  +  NH3 → Cl− + NH4+HCl\ \ +\ \ NH_3\ \rightarrow\ Cl^-\ +\ NH_4^+


  1. If we react a strong acid/base with a weak base/acid and have less of the strong substance then there will be a chemical reaction and there will be a buffer since the strong substance will completely react, leaving us with the weak substance and its conjugate=buffer Example:
HCl  +  NH3 → Cl− + NH4+HCl\ \ +\ \ NH_3\ \rightarrow\ Cl^-\ +\ NH_4^+


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There are 2 methods of preparing buffers:

  1. If sources of both HA and A- are readily available, then an appropriate amount of each component can be measured out to give the buffer solution.
  2. If only one of the conjugate acid-base pair is available (i.e. either HA or A–), a controlled amount of a strong acid or a strong base can be added to the solution of the available reagent which will result in the formation of HA and A– needed to achieve the desired buffer ratio.


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Example: Identifying Buffers

Determine if the following mixtures would constitute as buffers.

a) A solution containing acetic acid and sodium acetate.


Acetic acid (CH3COOH) and acetate (CH3COO-) are a conjugate acid/base pair, and therefore a mixture of the two would constitute as a buffer.


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b) A solution containing hydrocyanic acid and ammonia.


While hydrocyanic acid (HCN) and ammonia (NH3) are a weak acid and weak base, respectively, they are NOT conjugates of each other, and therefore this mixture would NOT constitute as a buffer.



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Examples: Forming Buffers

Do the following combinations of molecules form a buffer solution?

a) 1M CH3COO- with 0.8M HCl

CH3COO-: acid/base:
base
, weak/strong:
weak

HCl: acid/base:
acid
, weak/strong:
strong

Forms buffer? (write out chemical equation)
CH3COO- + H3O+ → CH3COOH + H2O
I 1mol 0.8mol 0 /
C -0.8 -0.8 +0.8 /
E 0.2M 0 0.8M /
Since we have both the weak acid and its conjugate base present, we can form a buffer!


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Do the following combinations of molecules form a buffer solution?

b) 2M HCl and 1M HBr

HCl: acid/base:
acid
, weak/strong:
strong

HBr: acid/base:
acid
, weak/strong:
strong

Would this combination form a buffer?
No, two acids cannot form a buffer. To have a buffer we would need a weak acid and its conjugate base present (or vice versa)

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Do the following combinations of molecules form a buffer solution?

c) 1M NH3 and 0.8M NH4+

NH3: acid/base:
base
, weak/strong:
weak

NH4+: acid/base:
acid
, weak/strong:
weak

Would this combination form a buffer?
Yes, because here we have a weak base and its conjugate acid present.


Extra Practice
Buffer Chemistry
Which of the following mixtures would constitute as a buffer.
Buffer Solutions
Consider the following diagrams representing different buffer solutions:
Which of these buffer solutions has the highest acid AND base buffer capacities?
Buffer Chemistry
Which of the following mixtures would constitute as a buffer.
Consider a buffer solution prepared from HOCl and NaOCl. Which is the net ionic equation for the reaction that occurs when NaOH is added to this buffer?
A buffer is formed by dissolving 5g of sodium benzoate, C6H5COONa, and 6g of benzoic acid, C6H5COOH (Ka =6.5 x 10-5) in 1 L of water.
Which of the following solutions can rightfully be called a buffer solution?
A solution made by mixing 1 mol of H2CO3 and 0.25 mol of NaOH
A solution made by mixing 1 mol of H2CO3 and 0.25 mol of K2CO3
Buffer Problem
What is the pH of the buffer that has 0.81M of CH3COOH and 0.21M CH3COO-? KCH3COOH=1.8x10^-5
Now if we added 0.14M of HCl what is the new pH of the buffer? (ICE table since we need to consider the values at EQUILIBRIUM to find the pH)
Buffer Solutions
Which of the following solutions can rightfully be called a buffer solution?
Buffers/Titrations
A student prepares a buffer solution in the lab by titrating bleach (sodium hypochlorite, NaCℓO) with HCl until they produce a 1:1 buffer. Ka for HClO = 3.5 x 10-8.