AP Chemistry



Honors Chemistry

Lab Activity: Buffering Action of Alka-Seltzer

Background: A buffer resists changes in pH when acid or base is added. Alka-Seltzer acts as a buffer by providing an antacid, citric acid, to neutralize stomach acidity and sodium bicarbonate to neutralize basicity.

When Alka-Seltzer dissolves in water these components react:

3NaHCO3 (aq) + H3C3H5O7 (aq) ( Na3C3H5O7 (aq) + 3CO2 (g) + 3H2O (l)

The sodium citrate acts as an antacid.

Na3C3H5O7 (aq) + HCl (aq) ( H3C3H5O7 (aq) + NaCl(aq)

The sodium bicarbonate acts to neutralize base.

NaHCO3 (aq) + NaOH (aq) ( Na2CO3(aq) + H2O (l)

Objective: The purpose of this activity is to test the buffering capacity of Alka-Seltzer. NaOH and HCl will be used to titrate an Alka-Seltzer tablet and plain water. A comparison will be made to determine which system will require more acid or base to achieve an acidic or basic pH.

Materials: (2)- 50 ml burets and buret stand

1M HCl solution

1M NaOH solution

Methyl Orange indicator

Phenolphthalein indicator

(2) - Alka-Selter tablets

(4) - 250 ml beakers

Stirring rod

Water bottle with distilled water

Procedures:

1. Fill one buret with 50 ml of HCl and the other with 50ml NaOH. Label each buret.

2. Label four beakers #1-4 and fill with 200 ml of distilled water.

3. Dissolve one Alka-Seltzer in beaker #2 and beaker #4

4. Add approximately 5 drops of methyl orange indicator to beakers # 1 &2

5. Add approximately 5 drops of phenolphthalein indicator to beakers # 3 & 4

6. Titrate beaker #1 with HCl while measuring pH, note the volume of HCl it takes to bring the water to a very low pH (pH≈1).

7. Titrate beaker #2 with HCl while measuring pH. Record the volume of HCl required to bring the solution to a very low pH ( monitor with color change).

8. Repeat procedures with beaker #3 & #4 using the NaOH solution and recording the volume NaOH required to raise the pH of the two samples to a high pH.

Data Table:

|Beaker Number |mL HCl |mL NaOH |

|1 H20 | |---- |

|2 Alka-Seltzer | |---- |

|3 H2O |---- | |

|4 Alka-Seltzer |---- | |

Post Lab Questions: Your answers should be written on a separate piece of paper and in complete sentences to receive full credit. Be sure to include your data table.

1. Why did the samples containing Alka-seltzer “consume” greater amounts of acid(HCl) and base (NaOH) than water and why did they eventually reach the same end point as water?

2. Why were methyl orange and phenolphthalein used as indicators in this study?

3. Based on your results, describe how Alka seltzer functions as a buffer.

4. Identify the compounds responsible for the buffering capacity of Alka seltzer ( i.e. weak acid/conjugate base).

5. Relate the properties of buffers to your understanding of Le Châtelier’s Principle.

Reference: Bassam Z. Shakashiri (1989) Chemical Demonstrations: A Handbook for Teachers of Chemistry, Volume 3. p 186-187.

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