Name:_______________________________________



Name:______________________________

Partner:____________________________

JHS Regents Chemistry Laboratory

Common Acid-Base Indicators

Objective:

Determine the pH of each of three unknown solutions by using a series of acid-base indicators.

Pre-Lab Activity & Questions:

Using colored pencils and Reference Table M, color the chart below to indicate the appropriate color from pH 1 to pH 14.

|Indicators |pH Values |

| |

Procedure:

1. Using the glass plate with 9 wells test each of the three unknown solutions with each of the indicators available.

2. When using an indicator, add only one drop to the well. If the color is too faded, add another until a color is distinguishable.

3. Write the color observed in the data table given. Be specific.

4. Determine the possible pH of each solution using Table M.

5. Wash the wells with soap and water. Rinse and dry.

Data:

COLORS AND pH RANGES OBSERVED BY INDICATORS

SOLUTION A SOLUTION B SOLUTION C

|Indicator |COLOR |pH RANGE |COLOR |pH RANGE |COLOR |pH RANGE |

| | | | | | | |

|BROMTHYMOL BLUE | | | | | | |

| | | | | | | |

|PHENOLPHTHALEIN | | | | | | |

| | | | | | | |

|LITMUS | | | | | | |

| | | | | | | |

|BROMCRESOL GREEN | | | | | | |

| | | | | | | |

|THYMOL BLUE | | | | | | |

**Test all solutions with one indicator before moving on to the next indicator so that color differences will be more clear!**

Conclusion:

| |Possible |Google search and name 2 substances that have a similar pH |

|Solution |pH | |

| | |___________________ |

|A | |___________________ |

| | |___________________ |

|B | |___________________ |

| | |___________________ |

|C | |___________________ |

If an unknown solution had a pH of 8, what colors would the following indicators have?

|Bromcresol Green |Litmus |Thymol Blue |

| | | |

Questions:

1. The human digestive system below is specialized for the digestion of different organic

compounds. To speed up the reaction that decomposes these compounds the digestive

system uses enzymes, which are organic catalysts that are proteins. Two important

enzymes are pepsin and trypsin. The drawing shows the pH of human mouth, stomach,

small intestine and large intestines. The graph shows the optimum pH values that the

enzyme reaction rate is highest.

a. Which part of the human digestive system would have the highest concentration of hydrogen ions? __________________________________

b. Referring to Table M, list all parts of the human digestive system where the indicator thymol blue would be yellow in color. ______________________________

c. State the approximate pH value where trypsin works at an optimum rate.___________________

2. Base your answers on the graph below. The graph shows the relationship between pH value and hydronium ion concentration for common aqueous solutions and mixtures.

a. What is the hydronium ion concentration of tomato juice? ___________

b. What is the hydroxide ion concentration of lemon juice? ___________

c. What color is litmus when added to seawater? ___________

d. According to the graph, which mixture is 100 times more acidic than milk of magnesia?

_______________________________

3. The active ingredient in the pain reliever aspirin is acetylsalicylic acid. This

compound can be produced by reacting salicylic acid with acetic acid. The label

of one aspirin bottle indicates that the accepted mass of acetylsalicylic acid in

each tablet is 325 milligrams. In a laboratory, an aspirin tablet is crushed and

mixed with water to dissolve all of the acetylsalicylic acid. The measured pH of

the resulting solution is 3.0.

a. Write the chemical formula for the acetic acid. ______________________

b. State the color of methyl orange indicator after the indicator is placed in the solution.

____________________

4. Vitamin C, also known as ascorbic acid, is water soluble and cannot be produced

by the human body. Each day, a person’s diet should include a source of

vitamin C, such as orange juice. Ascorbic acid has a molecular formula of

C6H8O6 and a gram-formula mass of 176 grams per mole.

a. What is the color of the indicator thymol blue after it is added to an aqueous solution

of vitamin C? ___________________

b. Determine the number of moles of vitamin C in an orange that contains 0.071 gram

of vitamin C.

b. Show a numerical setup for calculating the percent composition by mass of oxygen

in ascorbic acid.

c. Write the empirical formula for ascorbic acid. ____________________

5. Some carbonated beverages are made by forcing carbon dioxide gas into a

beverage solution. When a bottle of one kind of carbonated beverage is first

opened, the beverage has a pH value of 3.

a. State, in terms of the pH scale, why this beverage is classified as acidic.

b. Using Table M, identify one indicator that is yellow in a solution that has the same

pH value as this beverage.

______________________

c. After the beverage bottle is left open for several hours, the hydronium ion

concentration in the beverage solution decreases to 1/1000 of the original

concentration.

Determine the new pH of the beverage solution. ________________

6. The diagram below represents the pH ranges in which selected aquatic organisms exist. The solid figures represent the pH ranges in which the organism thrives. Shaded figures represent pH ranges in which conditions are less favorable but in which the organism survives. No symbol is placed in pH ranges in which organisms cannot survive. Use the diagram to answer the following questions.

a. Determine the pH in which brook trout can survive. ________________________

b. Which organism might be the best indicator that the pH of a lake has changed from 5.8 to 5.2? Explain your answer.

Organism: _____________________

Explanation:

________________________________________________________________________________________________________________________________

c. Name an organism that can tolerate a large range of pH conditions

________________________________________

d. Name an organism that can tolerate only a small range of pH conditions.

________________________________________

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