Experiment_617_pH_Acids_and_Bases_1_1_3

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Student Name

Laboratory Date:

Date Report Submitted:

___________________________

Student ID

Experiment Number and Title

Experiment 617: pH, Acids, and Bases

Section 1: Purpose and Summary

Understand relationship between pH and H⁺ ion concentration
Understand pH differences of acids and bases
Determine the pH of common solutions
Show how to use a pH indicator

A pH value is a number, usually between 0 and 14, that provides information about how acidic or basic a solution is. The “pH” is always written with a lowercase “p” and an uppercase “H”, which stands for “power of hydrogen.” pH values are related to hydrogen ion (H⁺) concentrations.

The mathematical relationship between pH and the molar concentration of H⁺ is described by the equation:

pH = - log [H⁺]

where log is the common log (the log of base 10) and [H⁺] is the molar concentration (i.e., moles of H⁺ per liter).

NOTE: It may be a good time to review logarithmic relationships. The fundamental ones you need to know are:

log_b (x) = y if b ^y= x and - log_b (x) = \(\frac{1}{\log _{b}(x)}\)

Because the negative of the log is being used, there is an inverse relationship between pH and [H⁺] ion concentration. As the [H⁺] concentration decreases, the pH value increases, and vice versa. If you are using common logs, the base of the logarithm is not usually shown.

When the pH value is a whole number (for example, a solution of pH 7), the whole number is equal to the negative exponent of the [H⁺] ion concentration.

Example: 10^-7 = [H+] = 10^-7 moles of H+ per liter of solution.

pH = -log (10^-7)

pH = - (-7)

pH = 7

The pH values of everyday chemicals typically range from pH 0 to pH 14. Values between 0 and 7 indicate an acidic solution. Values between 7 and 14 indicate a basic solution. A pH of exactly 7 indicates that a solution is neutral; it is neither acidic nor basic.

Pure water is usually pH 7. The pH scale is shown below.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

acidic neutral basic

(alkaline)

The lower the pH value, the more acidic the solution; the higher the pH value, the more basic the solution. Basic solutions are also called alkaline solutions. The pH scale can extend beyond 0 and 14, but these values are found in extreme cases. Strong, concentrated laboratory acids can have pH values less than 0 (negative pH values) and strong, concentrated laboratory bases can have pH values greater than 14. Solutions at extreme pH values are considerably more reactive in general and must be used with additional care.

The concept of pH is widely used in all areas of science including agriculture, biology, engineering and medicine. Many commercial products use pH as an advertisement tool, such as shampoo and water. Recently, food and drinks of certain pH has been touted as more healthful.

A pH indicator is a substance that, when a small amount of it is added to a solution of unknown pH, will have a color that depends upon the pH. This is a way to determine pH of a solution visually. pH indicators are a good way to easily and quickly show the approximate pH by color when compared to a standard. An everyday example where a pH indicator is used is for testing a water sample from a swimming pool. In the following table, you can see a variety of indicators with the approximate pH range over which each changes color.

Indicator	pH range	Color change
Methyl violet	0.0 to 1.6	Yellow to violet
Cresol Red (1)	0.2 to 1.8	Red to yellow
Thymol blue (1)	1.2 to 2.8	Red to yellow
Cresol purple (1)	1.2 to 2.8	Red to yellow
Methyl orange	3.1 to 4.4	Red to yellow
Methyl red	4.8 to 6.0	Red to yellow
Bromocresol green	3.8 to 5.4	Yellow to blue
Litmus	5.0 to 8.0	Red to blue
Bromothymol blue	6.0 to 7.6	Yellow to blue
Phenol red	6.4 to 8.0	Yellow to red
Cresol red (2)	7.2 to 8.8	Yellow to red
Cresol purple (2)	7.6 to 9.2	Yellow to purple
Phenolphthalein	8.0 to 9.6	Colorless to pink
Thymolphthalein	8.3 to 10.5	Colorless to blue
Thymol blue (2)	8.0 to 9.6	Yellow to blue
Alizarin yellow	10.1 to 12.0	Yellow to orange-red

Three indicators in the table above (cresol red, thymol blue and cresol purple) change colors in two different pH ranges. Indicators are usually identified by the common name.

1. Which indicator(s) in the table above would be useful to estimate the pH range of a strongly acidic solution?

Which indicator(s) in the table above would be useful to estimate the pH range of a strongly basic solution?

Which indicator(s) in the table above would be useful to estimate the pH range of a more neutral solution?

2. Thymol blue changes colors in 2 different pH ranges. If the solution you test with the thymol blue indicator turns yellow, would you be able to conclude whether that solution is acidic or basic? Why or why not?

3. You are given an unlabeled test tube and you need to find an approximate pH range of the solution in the tube. You are given the following data:

Indicator	Indicator color in solution
Cresol Red	Yellow
Methyl orange	Yellow
Phenolphthalein	Pink

What is the pH range you can identify for the solution in the unknown test tube? What evidence leads you to this conclusion?

Section 2: Safety Precautions and Waste Disposal

Safety Precautions:

Wear your safety goggles.

If any acid or base solution splashes on you, rinse it off immediately and notify your laboratory instructor. If any acid or base solution spills on a work surface, immediately notify your instructor.

Waste Disposal:

Waste from this experiment may be safely discarded down the drain using plenty of running water.

Section 3: Procedure

It is important to keep solutions from cross contaminating each other during this experiment. The best technique is to use a glass stir rod to get a drop of solution. You can rinse off the stir rod with laboratory water and dry it between tests.

Part 1: Estimating the pH of an Aqueous Solution of Nitric Acid (HNO₃)

Label each of 3 test tubes with a number #1, #2, or #3.

Obtain 9 ml of a 0.01M nitric acid (HNO₃) solution in a clean, dry 10-ml graduated cylinder. Pour 3 ml of the solution into each of your numbered test tubes. Use a test tube rack or stand to hold your test tubes upright.

Rinse the graduated cylinder twice with 5 ml of tap water each time followed by 5 ml of laboratory water.

Place one strip of blue litmus paper and one strip of red litmus paper on a clean, dry watch glass. Keep the two pieces of litmus paper well separated on the watch glass.

Dip the tip of a clean glass stirring rod into the (HNO₃) solution in one of your test tubes. Transfer that drop to the strip of blue litmus paper by touching the rod with the drop to one end of the blue litmus paper. Note the color and record your observations on Data Sheet line 1.

Repeat step 5 but transfer the drop from the stirring rod to the strip of red litmus paper. Note the color and record your observations on Data Sheet line 2.

Discard the used litmus paper in the garbage and rinse the stirring rod with laboratory water.

8. Add 3 drops of thymol blue indicator solution to the solution in test tube #1. Note the color, if any, and record your observations on Data Sheet line 3.

9. Add 3 drops of methyl orange indicator solution to the solution in test tube #2. Note the color, if any, and record your observations on Data Sheet line 4.

10. Add 3 drops of phenolphthalein indicator solution to the solution in test tube #3. Note the color, if any, and record your observations on Data Sheet line 5.

11. Discard the contents of your test tubes down the drain with plenty of running water. Rinse the test tubes twice with 5 ml of tap water followed by 5 ml of laboratory water. Dry the test tubes (and keep the labels or re-label as needed) to use in Part 2.

Part 2: Estimating the pH of Solutions of Various Substances

Note: Your laboratory instructor will assign solutions from the following list for you to study.

Soda

Shampoo

Ammonia cleaner

Bleach

Laundry detergent

Lemon juice

Vinegar

Bottled water

Obtain a 9 ml portion of the assigned solution in a clean, dry 10-ml graduated cylinder. Pour 3 ml of the solution into each of your dry, numbered test tubes.

Test the solution in one of the test tubes with blue litmus paper and red litmus paper following the same procedure as in Part 1 lines 5 and 6. Record your observations on Data sheet column 6-7 next to the formula of your assigned solution.

Add 3 drops of thymol blue indicator solution to the solution in test tube #1. Record your observations on Data sheet column 8 next to the formula of your assigned solution.

Add 3 drops of methyl orange indicator solution to the solution in test tube #2. Record your observations on Data sheet column 9 next to the formula of your assigned solution.

Add 3 drops of phenolphthalein indicator solution to the solution in test tube #3. Record your observations on Data sheet column 10 next to the formula of your assigned solution.

Discard your solutions and clean and dry your test tubes as you did in Part 1 line 11.

Repeat Part 2 steps 1-6 for any remaining assigned solutions and record your data in columns 9-13 next to the solution you tested.

Part 3: Data Tables

pH of Nitric Acid (HNO₃)	Observations
Blue litmus
Red litmus
Thymol blue
Methyl orange
Phenolphthalein

pH of various substances	Observations
0.1M solutions	Blue litmus (6)	Red litmus (7)	Thymol blue (8)	Methyl orange (9)	Phenolphthalein (10)
Soda
Shampoo
Ammonia cleaner
Bleach
Laundry detergent
Lemon juice
Vinegar
Bottled water

Is an aqueous solution of HNO₃ acidic or basic? What evidence supports your conclusion?

List each of the tested substances that are acidic. What evidence supports your selections?

List each of the tested substances that are basic. What evidence supports your selections?

Arrange the tested substances in order from most acidic to most basic.

Post Lab Question:

The labels fell off of three test tubes, each containing a colorless solution. A student knew that one of the test tubes contained a 0.1M HCl solution (acid), another contained a 0.1M NaOH solution (base), and the third contained a 0.1% phenolphthalein solution (indicator). To determine the identity of each solution, the student temporarily labeled the test tubes “A”, “B”, and “C”. The student mixed the solutions in various combinations, recording the observations in the table below:

Observations after mixing three unknown solutions

Mixture observations

1 drop of “A” + 1 drop of “B” red solution

Solution from (1) + 1 drop of “C” solution became colorless

Solution from (2) + 1 drop of “B” solution turned red

Determine which original label (with chemical identity) should be attached to each of the test tubes. Justify your answer briefly.

Notes: