Identifying Acids and Bases Lab

Purpose: To estimate the pH of common household substances using acid-base indicators. To mathematically determine the pH of anacidic and basic solution using pH=-log[H3O+] or pOH=-log[OH-].

Safety: Wear goggles and aprons at all times. Use care when working with chemicals.

Background: If you are interested in gardening, you may know that plants such as azaleas and rhododendrons prefer acidic soils. A part of soil testing consists of the measurement of the pH of the soil. pH is important for many other applications in the home and industry. For example, maintenance of the proper pH of water is important in caring for swimming pools. There are several ways to test the pH. One of the most common ways is by means of acid-base indicators such as phenolphthalein. In neutral and acidic conditions, phenolphthalein is colorless. In basic solutions, it is pink. Other acid-base indicators change colors at different pH values. By systematically testing a solution with a series of indicators, you can arrive at a good estimate of the pH of a solution.

Prelab Questions:

______1. What does the pH scale tell you about the concentration of the solution?

______2. Is hydrochloric acid a strong or weak acid?

______3. What is the pH of a 6.0M hydrochloric acid solution? Show work:

Procedures:

1. As you perform the experiment, record your observations in correct Data Table. Test each solution listed in Data Tables 1, 2, and 3 with one of the 6 indicators (Phenolphthalein, bromcresol green, bromothymol blue, methyl red, bromphenol blue, or litmus paper) to determine the pH. For each Data Table do not use the same indicator more then once!

2. To test a solution place a small amount into a clean test tube and place a few drops of the desired indicator in the test tube as well. Swirl test tube to mix contents. Record final color and predicted pH in Data Table. Use Table 16-6 on page 495 of a Modern Chemistry textbook and the color of the solution to predict the pH of the solution.

3. Rinse test tube and test another solution. Continue until you have tested all solutions in each Data Table. All solutions are safe to poor down the drain. Clean up your area and put everything back where you found it! Write the name of your favorite cartoon character next to Data Table 1.

Conclusion Questions:

______1. Can an acid-base indicator give you an exact value for the pH of a substance? Why or why not?

______2. What is the pH of a 1.5M NaOH solution? Show work:

______3. What is the pH of a 1.5M HCl solution? Show work:

______4. Use Kw to determine the [H+] and [OH-] for a 2.0X10-2M HCl solution. Show work:

Data Table 1: Indicators of pH in Various Solutions

Solution / Test or Indicator Used (Circle One) / Test Observations
(Color solution turned when indicator was present) / Predicted pH / Is the solution an acid or base?
HCl / Phenolphthalein bromcresol green
bromothymol blue methyl red
bromphenol blue litmus paper
NH3 (Ammonia) / Phenolphthalein bromcresol green
bromothymol blue methyl red
bromphenol blue litmus paper
NaOH / Phenolphthalein bromcresol green
bromothymol blue methyl red
bromphenol blue litmus paper
HC2H3O2
(Vinegar) / Phenolphthalein bromcresol green
bromothymol blue methyl red
bromphenol blue litmus paper

Data Table 2: Indicators of pH in Various Household Chemicals

Household Chemical / Test or Indicator Used (Circle One) / Test Observations
(Color solution turned when indicator was present) / Predicted pH / Is the solution an acid or base?
Phenolphthalein bromcresol green
bromothymol blue methyl red
bromphenol blue litmus paper
Phenolphthalein bromcresol green
bromothymol blue methyl red
bromphenol blue litmus paper
Phenolphthalein bromcresol green
bromothymol blue methyl red
bromphenol blue litmus paper
Phenolphthalein bromcresol green
bromothymol blue methyl red
bromphenol blue litmus paper
Phenolphthalein bromcresol green
bromothymol blue methyl red
bromphenol blue litmus paper
Phenolphthalein bromcresol green
bromothymol blue methyl red
bromphenol blue litmus paper

Data Table 3: Unknowns

Unknown Solution # / Test or Indicator Used (Circle One) / Test Observations
(Color solution turned when indicator was present) / Predicted pH / Is the solution an acid or base?
Unknown #1 / Phenolphthalein bromcresol green
bromothymol blue methyl red
bromphenol blue litmus paper
Unknown #2 / Phenolphthalein bromcresol green
bromothymol blue methyl red
bromphenol blue litmus paper
Unknown #3 / Phenolphthalein bromcresol green
bromothymol blue methyl red
bromphenol blue litmus paper