Experiment 7: PREPARATION AND PROPERTIES OF A BUFFER SOLUTION Post-Lab Questions

Question

Experiment 7: PREPARATION AND PROPERTIES OF A BUFFER SOLUTION Post-Lab Questions 1. What reaction is taking place when aqueous NaOH is added to a buffer so that the pH does not show a sharp increase? What reaction is taking place when aqueous HCl is added to a buffer so that the pH does not show a sharp decrease? Answer in full sentences and also formulas, HA and A-. Use the actual formula of chloroacetic acid. 2. Adding the NaOH and the HCl solutions dropwise to the buffer solution allowed you to see their effect on the pH of the buffer, what can you conclude about the effectiveness of the buffer? The purpose of Part II was to show how addition of a strong acid and of a strong base does not significantly affect the pH of a buffer. What role does the tap water (in Part III) play? Explain fully. 3. OAC

Explanation / Answer

Q1

A buffer is any type of substance that will resist pH change when H+ or OH- is added.

This is typically achieved with equilibrium equations. Both type of buffer will resist both type of additions.

When a weak acid and its conjugate base are added, they will form a buffer

The equations:

The Weak acid equilibrium:

HA(aq) <-> H+(aq) + A-(aq)

Weak acid = HA(aq)

Conjugate base = A-(aq)

Neutralization of H+ ions:

A-(aq) + H+(aq) <-> HA(aq); in this case, HA is formed, H+ is neutralized as well as A-, the conjugate

Neutralization of OH- ions:

HA(aq) + OH-(aq) <-> H2O(l) + A-(aq) ; in this case; A- is formed, OH- is neutralized as well as HA.

Q2

The buffer is very effective when neutralizing H+ and OH- if it is near to the pKa value

Q3

tap water will increase ions in solution

ions in solutoin increase the total ionic force, stregnht in solution

this is directly proportional to activities

activity of H+ will be affected, therefore, pH could change slightly more due to ions in solution

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