South Pasadena • AP Chemistry
Name
5 ▪ Acid Base Reactions
Period
Date
5 . 2 ALKA SELTZER™ DEMO/LAB
Equipment
8 plastic cups
Alka Seltzer tablet
1 M HCl
1 M NaOH
two glass stir rods
vinegar
universal indicator
phenolphthalien
Part I: pH Change of Distilled Water Demonstration
1. Pour boiled distilled water into the four cups until they are about half-filled.
To each cup add 30 drops of universal indicator. Stir. Set two cups aside for Part II.
Add a drop of 1 M HCl to one cup. Stir. Can you observe any color (pH) change? Repeat.
Add a drop of 1 M NaOH to the other cup. Stir. Do you observe any color (pH) change? Repeat.
2. Write your observations about how easily the color changes with added acid or base:
Part II: pH Change of Alka Seltzer Solution Demonstration
3. Break an Alka Seltzer tablet into two halves.
Add one half to each of the two cups with distilled water and universal indicator set aside in Part I.
Add a drop of 1 M HCl to one cup. Stir. Can you observe any color (pH) change? Repeat.
Add a drop of 1 M NaOH to the other cup. Stir. Do you observe any color (pH) change? Repeat.
4. Write your observations about how different the Alka Seltzer solution acts compared to water.
A solution that acts this way is called a ___________.
This behavior of the solution could be an operational definition for this type of solution.
Part III: Why Does This Happen?
Alka Seltzer contains the weak acid, citric acid, HC6H7O7, which can dissociate according to the following
equation:
HC6H7O7  H+ + C6H7O7
5. HC6H7O7 is a proton ________ (donor / acceptor)
C6H7O7 is a proton ________ (donor / acceptor)
6. Using Le Châtelier's Principle, indicate the effect of the following changes:
HC6H7O7  H+ + C6H7O7
add H+
___
___
___

add OH
___
___
___
7. In Alka Seltzer, the lack of color change is due to the fact that the added H+ ions are almost all mopped up
and the removed H+ ions are almost all replaced. This is Le Châtelier’s Principle carried to an extreme.
Added H+ ions are almost all absorbed by _____________.
H+ ions (removed by OH) are almost all replaced by ____________.
The solution contains large numbers of donors (weak acid) and acceptors (conjugate base).
HC6H7O7

H+ +
C6H7O7
A buffer can be thought of as a solution with a large concentration of acid (proton donor) and a base (proton
acceptor) that do not neutralize each other. This only occurs with a conjugate acid/base pair.
Part IV: Let’s Make A Vinegar Buffer (a.k.a. “Half Titration”) Demonstration
One way to make a buffer is to mix an acid with its conjugate base: HC2H3O2 + Na___________.
This could be accomplished by adding solid sodium acetate to a solution of acetic acid.
8. A second way to make a buffer is to neutralize some of the acid. Neutralizing changes an acid into its
conjugate base. In this case HC2H3O2 is changed into its conjugate base, ______________ .
Write the neutralization equation for HC2H3O2 with NaOH. Circle HC2H3O2’s conjugate base.
9. We will do this with a “half titration”, that is, we will neutralize HALF of an acid sample and then mix them
back together to make a buffer.
 Take two new plastic cups and add 10.0 mL of vinegar to each cup.
 Add two drops of phenolphthalein indicator to one cup of vinegar and neutralize the acid with NaOH.
In the cup of neutralized acid, essentially all of the HC2H3O2 has been converted to __________.
 Mix the contents of the two cups together. This solution should be a buffer.
10. Add universal indicator to the solution until you get a significant amount of color. From the color, would you
say the solution is acidic, basic, or neutral? _____________ (Come back to this observation in Step 20.)
Add boiled distilled water until the cup is 2/3 full. Add a little more universal indicator. How does the color
of the buffer change with the addition of water? ______________________
11. Pour a small amount of the buffer solution into a clean cup and save for Part V.
12. Split the solution from Step 10 into two cups and test the solution as you did in Parts I and II to see if the
solution is a buffer. How many drops of acid/base does it take to change the solutions to the colors attained
with 1 drop of HCl and 1 drop of NaOH in distilled water? _____ drops HCl _____ drops NaOH
Is the solution a buffer?
13. Write two “recipes” for making a buffer beginning with 1 mole of acetic acid:
 1 mole HC2H3O2 + 1 mole __________ will make a buffer.
 1 mole HC2H3O2 + 0.5 mole __________ will make a buffer.
Part V: Buffering Capacity
Take the small amount of buffer from Step 11 above. Fill the cup 2/3 full with boiled distilled water. Add more
universal indicator to the solution to increase the color. Split the solution between the two cups. Test the two
samples to see whether they act as a buffer.
14. How many drops of acid/base does it take to change the solutions to the colors attained with 1 drop of HCl
and 1 drop of NaOH in distilled water? _____ drops HCl _____ drops NaOH
Write your observations about the “buffering capacity” of the diluted buffer:
Part VI: Questions and Calculations
15. The equilibrium of a buffer is the same as the equilibrium of the dissociation of an acid. Write the dissociation
equation for HC2H3O2. Ka = 1.8 × 10−5.
16. In your mixture (Step 10 above), how does [HC2H3O2] compare to [C2H3O2]? ____________
17. Write the Ka expression for HC2H3O2.
18. If [HC2H3O2] = [C2H3O2], the expression changes to:
19. What is the significance of the pKa?
20. The pH of this buffer must be _______ (Relate to color observed in Step 10 above.)
21. In Step 10, it was observed that adding water did not change the color of the solution (only diluted it a little).
Explain this observation in terms of the ratio of [HC2H3O2] to [C2H3O2].
22. The most effective buffer is made from large equal concentrations of the acid and its conjugate base. In this
case, the pH = pKa. What is the pH of the best buffer made from the following acids?
(a) HCN
Ka = 6.2 × 10−10
The pH of the best buffer is _____.
(b) HF
Ka = 7.2 × 10−4
The pH of the best buffer is _____.
−8
(c) HClO Ka = 3.5 × 10
The pH of the best buffer is _____.
23. For HC2H3O2, the pH of the best buffer is: ______ If you wanted a buffer with a pH of 5.00, however, would
you need to add more acid, HC2H3O2, or more base, C2H3O2? ______
24. Determine the ratio of [C2H3O2] to [HC2H3O2] that would form a buffer with a pH of 5.00. _______