Buffer experiment

Question

20 ml of sodium acetate( buffer ) was poured into beaker 1, and 20 ml of distilled water was poured into beaker 2. BTB indicator was added to both beakers. The color and pH were observed. The beaker containing sodium acetate turned yellow and had a pH of 3. The beaker containing distilled water turned green and had a pH of 7. Then two drops of diluted hydrochloric acid was added to both beakers. The beaker containing sodium acetate remained yellow and had a pH of 3. The beaker containing distilled turned yellow and had a pH of 3.

Questions:

Before HCl was added to the sodium acetate solution, the color was yellow and the pH was 3, when hydrochloric acid was added to the beaker containing containing sodium acetate, the solution resisted change in color ( yellow ) and pH ( 3 ). Why.

Before HCl was added to the distilled water solution, the color was green and the pH was 7, when hydrochloric acid was added to the beaker containing distilled water, the solution changed color from green to yellow and its pH from 7 to 3. Why.

Write the reaction formula for the two solutions, showing what happened when the acid was added.

What’s the point of showing the reaction formulas?

Answer 1

When you say you had a sodium acetate BUFFER, that implies the solution contained sodium acetate AND acetic acid.  That is was it takes to make a buffer (a weak acid + the salt of that acid).

Question 1:  When HCl is added to this buffer, a change is pH is resisted because the added H+ reacts with the sodium acetate to form the WEAK acid, acetic acid.  Since this is a weak acid, it does not dissociate to any appreciable extent, and so it "essentially" removes the added H+ from solution, and the pH doesn't change appreciably.  

 

Question 2:  Distilled water cannot resist the change in pH because any added H+ simply combines with H2O to make H3O+ and the more H3O+ you have, the lower the pH.

 

Question 3:  Sodium acetate = CH3COONa = CH3COO^- + Na+ and Na+ is a spectator

Acetic acid = CH3COOH

Buffer = CH3COOH + CH3COO^- mixture

When HCl is added, it will react with the CH3COO^- as follows:

CH3COO^- + H⁺ ==> CH3COOH and Cl^- is a spectator along with Na⁺

 

Question 4:  When you write the reaction, you can easily see that the added acid reacts with the acetate anion (conjugate base of acetic acid) forming the WEAK acid, acetic acid.