Question

Answer these 2 questions

1. Predict how a buffer of HA and A- will resist change in pH when a strong acid is added. Write down a reaction to represent your prediction.

2. Acetic acid is CH3COOH, the Ka for acetic acid is 1.8x10-5.  

   1. Write the dissociation reaction for acetic acid.

   2. Calculate the pH of a 0.240 M acetic acid solution.

   3. Why would sodium acetate need to be added to acetic acid to create a buffer?

   4. Once the buffer of acetic acid and sodium acetate is made write a reaction showing how the buffer resist a change in pH when HCl(aq) is added.

   5. Once the buffer of acetic acid and sodium acetate is made write a reaction showing how the buffer resist a change in pH when NaOH(aq) is added.

Answer 1

• Buffer is made of a solution of weak acid & its salt of strong base.

Now, say , HA is a weak acid & A- is its salt of strong base. When a strong acid is added to their buffer solution ,the conjugate base (A-) gets protonated . But as it is a weak acid , it does dissociate slightly in comparison to protonation.

Therefore , the pH of the solution does not change so much ,rather be unchanged though in between a certain range.

A- + H+ = HA (persists as undissociated or mildly dissociate.)

• CH3COOH + H2O = CH3COO- + H3O+
• pH of the solution of a weak acid can be written as (Kac)(1/2).

pH=-[ (1/2)(1.8) + (1/2)(-5) +(1/2)0.24] = 2.68

• sodium acetate have to be added to increase the concentration of conjugate base which plays the key role to resist the increment of H+concentration due to addition of strong acid.
• CH3COO-  + H+ = CH3COOH (mildly dissociate)
• CH3COOH + OH- = CH3COO- + H2​​​​​​​O