Materials Included In Kit

Additional Materials Required

Prelab Preparation

Please read all steps before starting Procedure.

1. Using a Beral-type pipet, add 25 drops of phenolphthalein to each of the three hydrochloric acid solution bottles—the 1 M, 0.5 M, and 0.1 M HCl. Recap the bottles and swirl the solutions to mix.
2. Use the permanent marker to label 60 pipets 1–60.
3. Using a separate sheet of paper, make a numbered list 1–60.
4. Obtain pipets labeled 1–4.
5. Organize the six mystery solutions for easy pipet filling: 1 M hydrochloric with phenolphthalein added from step 1, 0.5 M hydrochloric acid with phenolphthalein added from step 1, 0.1 M hydrochloric acid with phenolphthalein added from step 1, 1 M sodium hydroxide, 0.5 M sodium hydroxide and 0.1 M sodium hydroxide.
6. Fill the first four pipets with at least 2 mL of four different solutions. Write the solution added to each pipet on the numbered list made in step 3. Note: Within each group of four pipets, none of the possible solutions may be duplicated.
7. When a pipet is filled, orient the pipet with its tip up and place in a beaker or disposable cup. When a set of four pipets is filled, the tips can be rubber-banded together to create a set for each student group (see Figure 1).
8. Fill the remaining groups of four pipets repeating steps 6–7 until all 15 groups of four pipets each have been filled and recorded.
9. Keep the reference list of unknowns in a secure place. This list may be needed to accommodate any pipet refills during the laboratory exercise and it will be needed for grading purposes.
   {12266_Preparation_Figure_1}

Safety Precautions

Sodium hydroxide solutions are corrosive to all body tissue, especially to the eyes. Hydrochloric acid is toxic by ingestion or inhalation and corrosive to all body tissues. Phenolphthalein solution is a flammable liquid and is toxic by ingestion and inhalation. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The excess hydrochloric acid may be disposed of by neutralizing with base such as the leftover sodium hydroxide solution and then disposed of down the drain with excess water according to Flinn Suggested Disposal Method #24b. The leftover sodium hydroxide solution may be disposed of by neutralizing with acid such as the leftover hydrochloric acid solution and then disposed of down the drain with plenty of excess water according to Flinn Suggested Disposal Method #10. The excess phenolphthalein may be stored for future use. The test solutions should be neutralized with either acid or base and disposed of down the drain according to Flinn Suggested Disposal Method #10 or #24b.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. This laboratory activity can reasonably be completed in one 50-minute class period. The prelaboratory assignment may be completed before coming to lab, and the data compilation and questions may be completed the day after the lab.
• The lab is written using six unknowns but a random four are given to each group. To make the lab easier, with two additional pipets per group all six solutions could be given to all groups.
• Some students may need to dispose of their test solutions while lab is still in progress. These test solutions may be held in a labeled beaker until all test solutions are generated and can be treated at the completion of the laboratory exercise.

Teacher Tips

• This experiment can be adapted for various levels of instruction. The lab can be performed as written for general chemistry classes. For more advanced classes, consider not giving the step-by-step procedure—just the six listed possible unknowns and the safety information.
• These solution groupings to be identified by the students have not been directly assigned. This has been left to instructor’s discretion who may want to vary the identity of the unknown for different groups and or classes, if applicable. Many teachers label the unknown samples randomly with no evident patterning with numbers or letters and maintain a key for grading purposes.
• This is a qualitative lab; therefore, exact amounts used in the tests are not crucial but the ratios are important to determine concentration.
• To conserve chemicals, a suggestion of micro-titrating is given: 1 drop to determine acid or base identification and 3 drops of an unknown solution to determine concentration.
• If you like this laboratory format and wish to try a similar activity, Match the Mystery Solutions—Guided-Inquiry Laboratory Kit is available from Flinn Scientific, Catalog No. AP7323. This kit encourages logical thinking and good communication between laboratory partners to determine the mystery solutions.

Answers to Prelab Questions

1. Read the entire Procedure section and the Safety Precautions. Why is it important to keep the tips of the pipets out of the test solutions and to use toothpicks to mix the solutions?

   Using toothpicks instead of the pipets to mix will help prevent contamination of the samples.

2. Write the balanced chemical equation for the reaction of hydrochloric acid with sodium hydroxide.

   HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O(l)

3. Phenolphthalein is an acid–base indicator. Complete the following table to classify solutions having a pH of 6 and 10.
   {12266_PreLabAnswers_Table_3}
4. For each set of four mystery solutions to be analyzed, none of the possible solutions will be duplicated. Under these parameters, what condition must be true for at least one pair of the solutions?

   At least one acid and one base have to be present in the mystery solutions.

5. Equation 4 can be used qualitatively to determine the relative concentrations of acid and base. If ten drops of 0.5 M HCl solution containing phenolphthalein are placed in a well, and 5 drops of NaOH solution are needed to neutralize the acid, what is the concentration of NaOH? Explain your reasoning.

   Ma(Drops)_a = M_b(Drops)_b
   (0.5 M HCl)(10 drops HCl) = M_bNaOH (5 drops NaOH)
   1 = M_bNaOH
   The concentration of the NaOH is 1 M NaOH.

Sample Data

Answers to Questions

1. Fill in the solution identities.
   {12266_Answers_Table_5}
2. Write a paragraph explaining how observations and data analysis identified all four unknown solutions.

   The first set of solutions had to be both acids or both bases since there wasn’t a reaction. Trying the third solution with solution 1 gave a pink result showing one must be an acid and one must be a base. By counting the drops the concentration ratio between the two could be determined. After determining the relative concentration of an acid–base pair it was easier to determine if the other two solutions were acids or bases and then from there the concentration ratios could be determined. Looking at the known listing of possible choices the ratios could be compared. A 1 M is 10 times as concentrated as a 0.1 M and twice as concentrated as a 0.5 M solution.

3. If two unknown solutions were tested, an acid and a base, could the concentration of both solutions be found regardless of which solution is titrated?

   The starting solution does not matter. When starting with the base, the solution will be pink and it will be necessary to count the drops to titrate to colorless. If the starting material is the acid, the solution will titrate from colorless to pink.

4. “Don’t mix chemicals unless instructed to do so” is a good general safety rule—unpredictable reactions may take place. The following are some common “chemicals” found in most homes.

   A. Vitamin C
   B. Baking soda
   C. Washing soda
   D. Epsom salts
   E. De-icing salt
   
   Compounds A–E are all white solids that are soluble in water. When the solids were dissolved in water and then mixed pairwise in a laboratory as shown in the following table, several reactions were observed (NR—no reaction; ppt—precipitate). Note that since mixing A + B has the same effect as mixing B + A, only half the table is filled in.

   {12266_Answers_Table_6}
   Assume someone removed the labels from the household substances and scrambled them—they are now called 1–5. Identify 1–5 based on the following data.
   {12266_Answers_Table_7}

References

“Minimal acid–base lab practical” Chem 13 News January 2010 p 4

Introduction

Identifying unknown substances often presents scientists with a mystery or puzzle that needs to be figured out. Determine the identity of different mystery solutions using of the scientific method.

Concepts

• Scientific method
• Acids and bases
• Concentration
• Titration

Background

The scientific method is a way of solving problems using a logical and systematic approach. When observing the world, conclusions are drawn based on observations and research. Curiosity brings forth a question. A possible answer or explanation to the question—called a hypothesis—is formulated. A hypothesis is a reasonable and educated possible answer based on what is known and what has been observed. Based on a series of observations, the hypothesis is tested by means of additional experiments designed to either validate or disprove the hypothesis. Scientific knowledge becomes more reliable as we gather more information and continuously test hypotheses. The scientific method is sometimes presented as a rigid sequence of events.

The scientific method, however, is not rigid; it is a process—a process of discovery! Discovery begins when we make observations and then try to understand what we have observed by asking clarifying questions and proposing possible answers. The process of discovery continues as we design and conduct experiments to test whether our answers to these questions are valid.

In this guided-inquiry experiment involving acid and base “mystery” solutions, deductive reasoning and the scientific method will be used to identify a series of four unknown substances. The following discussion of the properties of acids and bases will help you devise a logical sequence or pathway for mixing the mystery solutions and determining the identity and concentration of each.

Each team of students will receive a unique set of four unknowns from the following list of six possible solutions. Notice that there are three concentrations of hydrochloric acid and three concentrations of sodium hydroxide (see Table 1).

All of the hydrochloric acid mystery solutions will also contain phenolphthalein, which has been added as an acid−base indicator. (See the following paragraph for a discussion of the use of an indicator to detect the endpoint in the reaction of an acid and a base.) Phenolphthalein is colorless in acidic solutions (pH <7) and pink in solutions having pH >8–10. As excess base is added to an acidic solution to raise the pH, the phenolphthalein indicator will turn pink.

Strong acids and bases, such as hydrochloric acid and sodium hydroxide, react to give neutral products that are neither acidic nor basic. The products of these neutralization reactions are water and an ionic compound, or salt, such as sodium chloride. The amount of acid that will react with a specific amount of base is governed by the mole ratio for the neutralization reaction. Neutralization reactions are frequently used in the laboratory to determine the concentration or amount of an acidic or basic compound in an unknown solution or in another substance. The technique of measuring the volume of base required to neutralize a given amount of acid (or vice versa) is called a titration. In this lab, microscale titrations of the unknowns will be carried out by counting the number of drops of one mystery solution that are needed to neutralize a specific number of drops of another mystery acid or base solution. Phenolphthalein has been added as an acid−base indicator in the mystery acid solutions (see above) to determine the endpoint of each titration—that is, the point at which the number of moles of acid equals the number of moles of base (Equation 1).

At the endpoint: The molar concentration (M) of a substance in solution is related to the number of moles by the definition for molarity (Equation 2). Rearranging Equation 2 to solve for moles, and substituting the result in Equation 1, gives the following equation that can be used to deduce the relative concentration of an acid or base, assuming they react in a 1:1 mole ratio (Equation 3). Since volume (V) appears on both sides of Equation 3, any unit of volume may be used, as long as the units for the acid and base are the same. For microscale titrations it is convenient to use Equation 4, in which the volumes are expressed in number of drops.

Experiment Overview

Four unknown or mystery solutions of acids and bases will be available for analysis. Through deductive reasoning and microscale titrations the identity and concentrations of the four unknown solutions can be determined.

Materials

Prelab Questions

1. Read the entire Procedure section and the Safety Precautions. Why is it important to keep the tips of the pipets out of the test solutions and to use toothpicks to mix the solutions?
2. Write the balanced chemical equation for the reaction of hydrochloric acid with sodium hydroxide.
3. Phenolphthalein is an acid–base indicator. Complete the following table to classify solutions having a pH of 6 and 10.
   {12266_PreLab_Table_2}
4. For each set of four mystery solutions to be analyzed, none of the possible solutions will be duplicated. Under these parameters, what condition must be true for at least one pair of the solutions?
5. Equation 4 can be used qualitatively to determine the relative concentrations of acid and base. If ten drops of 0.5 M HCl solution containing phenolphthalein are placed in a well, and 5 drops of NaOH solution are needed to neutralize the acid, what is the concentration of NaOH? Explain your reasoning.

Safety Precautions

Sodium hydroxide solutions are corrosive to all body tissue, especially to the eyes. Hydrochloric acid is toxic by ingestion or inhalation and corrosive to all body tissues. Phenolphthalein solution is a flammable liquid and is toxic by ingestion and inhalation. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

Procedure

1. Obtain a set of four labeled pipets containing unknown acid and base solutions.
2. On the data sheet under Question 1 list the numbers of the four unknowns.
3. Dispense 1 drop of one of the unknowns into a well of a reaction strip.
4. Take another pipet containing a different unknown. Add the solution dropwise to the reaction well containing the solution from step 3 until a color change is observed or until 11 drops of the second solution have been added.
5. Mix the solution as needed with a toothpick. Note: Do not mix with the pipet or touch the pipet tip to a solution—it will contaminate the solution.
6. Record observation and possible conclusion in the data table.

Repeat steps 3−6 with different solutions based on the following principles:

7. Continue titrating drops of unknown solutions pairwise (e.g., 1 and 2, 1 and 3, 2 and 3) in separate wells on the reaction strip until you can deduce whether each solution is acidic or basic and can determine their relative concentrations.
8. After each step write a possible conclusion (e.g., both 1 and 2 must be acids, or 1 and 2 must be an acid and a base, but we don’t know which is which), and then brainstorm another testing combination you could use to verify if the conclusion is valid.
9. No single test will give you all the information you need to solve the puzzle of the mystery solutions! You must continue to adapt the combinations you use based on information from the previous test. Remember to count the number of drops!
10. Hint: Use one drop of the first unknown in a well when trying to identify the type of solution, and use three drops of the first unknown when trying to determine the concentration.
11. If all of the wells on the reaction strip have been used, empty the contents of the strip into a small beaker or waste cup. Rinse the wells with plenty of distilled or deionized water from a wash bottle before reuse.

Student Worksheet PDF

12266_Student1.pdf