Question

Property	Flask 1	Flask 2	Flask 3	Flask 4
Contents	100 ml water	100 ml water	74ml acetic acid,1.1g sodium acetate, 26ml water	74ml acetic acid, 1.1g sodium acetate,26 ml water
Initial pH	7.00	7.00	5.01	5.01
pH after adding strong acid(HCL)	2.32		4.97
pH after adding strong base(NAOH)		11.68		5.06

1. Compare what happen to the pH of flask 1 to what happened to the pH of flask 3 when HCl was added.

2. Which substance, water or the buffer does a better job of maintaining pH when small amounts of strong acid are added?

3. Compare what happen to the pH of flask 2 to what happened to the pH of flask 4 when NaOH was added.

4. Which substance, water or the buffer does a better job of maintaining pH when small amounts of strong base are added?

5. Write equations for the reactions taking place in each of the flasks. For an equilibrium arrow use equals sign (=) ( use H2O for water, A- for acetate ion, HA for acetic acid, H+ for hydrogen ion, H3O+ for hydronium ion, Na+ for sodium ion, and OH- for hydroxide ion.

a. Reaction in flask 1

b. Reaction in flask 2

c. Reaction in flask 3.

d. Reaction in flask 4.

6. In your own words, tell how this experiment is related to the buffer systems in the blood.

Answer 1

1) In flask one, after the addition of strong acid HCl, the pH drops to 2.32. However, the acetate buffer in the flask 3 maintains the initial pH around(4.9).(Note that a buffer solution, which is the mixture of weak acid and its conjugate base or vice versa- maintains the pH even after the addition of small amounts of acid or base)

2) Obviously, the buffer does a better job of maintaining the pH, when a small amount of acid or base is added. The extent to which a buffer can resist the change in pH depends upon the particular buffer and is related to its buffer capacity.

3) When NaOH, which is a strong base is added to water, the pH of the water increased to 11.68. Here also, the acetate buffer in the flask 4 maintained the initial pH

4) Here also buffer does a better job of maintaining the pH

5)

a) H₂O + HCl H₃O+ Cl^-

b) H₂O + NaOH Na⁺+ OH^- + H₂O

c) H⁺+ A^- HA + H₂O

d) OH^- + HA H₂O + A^-

6) The blood is a buffer system of carbonic acid(H₂CO₃) and bicarbonate(HCO₃^-). This buffer system maintains the blood pH between 7.35 and 7.45. In the blood buffer, the bicarbonate anion and hydronium ions are in equilibrium with the carbonic acid