Materials Included In Kit

Additional Materials Required

Safety Precautions

Hydrochloric acid is toxic by ingestion or inhalation and is severely corrosive to skin and eyes. Phenolphthalein solutions contain alcohol and are flammable liquids; they are toxic by ingestion. Do not use near flames or other sources of ignition. Dilute sodium hydroxide solution is slightly toxic by ingestion and skin absorption and is irritating to skin and eyes. Dilute potassium hydroxide solution is a skin and body tissue irritants and slightly toxic by ingestion. Avoid contact of all chemicals with eyes and skin. 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. Excess acid may be neutralized according to Flinn Suggested Disposal Method #24b. Excess base may be treated according to Flinn Suggested Disposal Method #10. The titrated solutions are considered neutral and may be rinsed down the drain with excess water according to Flinn Suggested Disposal Method #26b.

Lab Hints

• This laboratory activity was specifically written, per teacher request, to be completed in one 50-minute class period. It is important to allow time between the Preab Homework Assignment and the Lab Activity. Once students turn in the homework answers, tables and figures and their procedure, check the procedure for safety and accuracy before allowing implementation in the lab. If gel capsules will be used, have students prepare their lab area the day before analyzing the mixture. Having the water trough filled with water and buret already filled with the titrant will help the lab go quicker on the second day. If the buret is filled ahead of time, cover the top with Parafilm^® to prevent evaporation.
• Remind students to be careful when titrating because they will only have one titration.
• Depending on your preference, you can either limit student choices to specific concentrations of the hydrochloric acid and sodium hydroxide solutions before the students write their procedures, or you may prepare more dilute solutions, based on your students’ procedures. When diluting acids, always remember to add acid to water.
• You can also limit the amounts of acid or base each group can use in their procedure. For example, each group may only be given 50 mL of 1.0 M NaOH. Enough materials are included in this kit for each group to use 100 mL of 1.0 M NaOH and 20 mL of 6.0 M HCl.
• For best results, prepare capsules with mixtures of the three potassium compounds ahead of time. You can place the capsules in baggies or small beakers and label with an unknown number. For your records, keep track of the mass of each individual potassium compound. With this info, this lab can then be used to grade students on how accurate their results are to the original data. Students will love the challenge!
• For the 3-g mixture samples, do not place more than one gram of potassium carbonate in the sample. This will allow the gas that is created to be collected in a 250 mL flask. When you prepare the samples, it may work best to let students know that potassium carbonate and potassium hydroxide are both only 33% of the mixture or less. This will allow your students to better estimate the amount of acid and base needed.
• When preparing capsules, weigh and add the KOH last. If it sits out too long, it will absorb moisture from the air and be more difficult to place inside the capsules.|Instead of gel capsules, thistle tubes can be used for a gas collection apparatus.
• If glass tubing must be inserted into a rubber stopper, use the Glass-a-Matic Hand Saver (Flinn Catalog No. AP4599). This should be done by the instructor before lab day. You can watch a video on how to insert glass tubing into a rubber stopper at the Flinn website (www.flinnsci.com).
• When preparing for a titration, students should rinse the buret with the titrant. Explain to students that rinsing the buret with only water will change the initial concentration of the titrant.
• If preferred, you could also have the students standardize their sodium hydroxide solutions to verify the initial concentration of the titrant.
• Remind students to read the volume in a buret from the top-down. A buret is marked every 0.1 mL and thus the volume may be estimated to two decimal places (see Figure 2).
  {12343_Hints_Figure_2}

Teacher Tips

• Quantitative analysis represents a nearly invisible application of chemistry in our daily lives. To illustrate the importance of quantitative analysis, ask students how they would feel if they could not trust that the water they drink or the medicines they take had been tested to assure quality and safety.
• Challenge your students to see who can get the closest results! This is a great challenge activity. Prepare different unknown mixtures for the groups. Record the initial masses of the three individual potassium compounds. When students finish the lab, compare their answers to the initial data.

Further Extensions

Alignment to the Curriculum Framework for AP^® Chemistry 

Enduring Understandings and Essential Knowledge
All matter is made of atoms. There are a limited number of atoms; these are the elements. (1A)
1A3: The mole is the fundamental unit for counting numbers of particles on the macroscopic level and allows quantitative connections to be drawn between laboratory experiments, which occur at the macroscopic level, and chemical processes, which occur at the atomic level.

Atoms are conserved in physical and chemical processes. (1E)
1E2: Conservation of atoms makes it possible to compute the masses of substances involved in physical and chemical processes. Chemical processes result in the formation of new substances, and the amount of these depends on the number and the types and masses of elements in the reactants, as well as the efficiency of the transformation.

Chemical changes are represented by a balanced chemical equation that identifies the ratios with which reactants react and products form. (3A)
3A2: Quantitative information can be derived from stoichiometric calculations that utilize the mole ratios from the balanced chemical equations. The role of stoichiometry in real-world applications is important to note, so that it does not seem to be simply an exercise done only by chemists.

Chemical reactions can be classified by considering what the reactants are, what the products are, or how they change from one into the other. Classes of chemical reactions include synthesis, decomposition, acid–base, and oxidation–reduction reactions. (3B)
3B2: In a neutralization reaction, protons are transferred from an acid to a base.

Chemical equilibrium plays an important role in acid–base chemistry and in solubility. (6C)
6C1: Chemical equilibrium reasoning can be used to describe the proton-transfer reactions of acid–base chemistry.

Learning Objectives
1.2 The student is able to select and apply mathematical routines to mass data to identify or infer the composition of pure substances and/or mixtures.
1.18 The student is able to apply conservation of atoms to the rearrangement of atoms in various processes.
1.20 The student can design, and/or interpret data from, an experiment that uses titration to determine the concentration of an analyte in a solution.
1.3 The student is able to select and apply mathematical relationships to mass data in order to justify a claim regarding the identity and/or estimated purity of a substance.
1.4 The student is able to connect the number of particles, moles, mass, and volume of substances to one another, both qualitatively and quantitatively.
3.3 The student is able to use stoichiometric calculations to predict the results of performing a reaction in the laboratory and/or to analyze deviations from the expected results.
3.4 The student is able to relate quantities (measured mass of substances, volumes of solutions, or volumes and pressures of gases) to identify stoichiometric relationships for a reaction, including situations involving limiting reactants and situations in which the reaction has not gone to completion.
3.7 The student is able to identify compounds as Brønsted-Lowry acids, bases, and/or conjugate acid–base pairs, using proton- transfer reactions to justify the identification.
6.11 The student can generate or use a particulate representation of an acid (strong or weak or polyprotic) and a strong base to explain the species that will have large versus small concentrations at equilibrium.
6.12 The student can reason about the distinction between strong and weak acid solutions with similar values of pH, including the percent ionization of the acids, the concentrations needed to achieve the same pH, and the amount of base needed to reach the equivalence point in a titration.
6.15 The student can identify a given solution as containing a mixture of strong acids and/or bases and calculate or estimate the pH (and concentrations of all chemical species) in the resulting solution.
6.16 The student can identify a given solution as being the solution of a monoprotic weak acid or base (including salts in which one ion is a weak acid or base), calculate the pH and concentration of all species in the solution, and/or infer the relative strengths of the weak acids or bases from given equilibrium concentrations.

Science Practices
1.2 The student can describe representations and models of natural or man-made phenomena and systems in the domain.
1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively.
2.1 The student can justify the selection of a mathematical routine to solve problems. (Appropriateness of selected mathematical routine.)
2.2 The student can apply mathematical routines to quantities that describe natural phenomena.
3.1 The student can pose scientific questions.
3.3 The student can evaluate scientific questions.
4.1 The student can justify the selection of the kind of data needed to answer a particular scientific question.
4.2 The student can design a plan for collecting data to answer a particular scientific question.
4.3 The student can collect data to answer a particular scientific question.
4.4 The student can evaluate sources of data to answer a particular scientific question.
5.1 The student can analyze data to identify patterns or relationships.
5.3 The student can evaluate the evidence provided by data sets in relation to a particular scientific question.
6.1 The student can justify claims with evidence.
6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices.
7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/or big ideas.

Answers to Prelab Questions

1. The ingredients of two different brands of baking powder are shown below. Research the ingredients and correctly label which ingredients act acidic or basic when used in cooking.

   Baking Powder A’s Ingredient List: Sodium acid pyrophosphate, Sodium bicarbonate, Corn starch and Monocalcium phosphate
   Baking Powder B’s Ingredient List: Corn starch, Sodium bicarbonate, Sodium aluminum sulfate, Monocalcium phosphate

   {12343_PreLabAnswers_Table_3}
2. According to the label, a sample of baking powder A is 29.3% sodium bicarbonate. A student decides to add 9.5 mL of 6 M hydrochloric acid to a 3.02-g sample of the baking powder. He then collects the gas over water. The temperature and pressure of the room are 20.0 °C and 0.99595 atm.
   1. Write the balanced chemical equation for the reaction occurring above.

      NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)

   2. The student collected the gas with the following setup. Label all the unlabeled equipment that was used in the experiment.
      {12343_PreLabAnswers_Figure_4}
   3. What gas was collected?

      Carbon dioxide and water vapor was collected.

   4. When the reaction finished and no more gas was produced, the student marked the gas/water line on the collection glass. The teacher emphasized to align the water line in the collection glass with the water line in the trough, before marking the collection glass. Why is this important?

      If the water lines are not aligned, then the pressure of the gas in the collection glass will be different than the room pressure. When the water lines are lined up, the pressure inside the collection glass is the same as the atmospheric pressure.

   5. If the student collects 251.5 mL of gas, what is the mass of sodium bicarbonate that reacted?

      P_total = 0.99595 atm
      P_CO2 = 0.99595 atm – 0.02309 atm
      P_CO2 = 0.97286 atm
      PV = nRT
      (0.97286 atm)(0.2515L) = n(0.08206 atm•L•mol^–1•K^–1)(20.0 + 273.15)
      n = 0.01017 moles CO₂
      0.01017 moles CO₂ x (1 mol NaHCO₃/1 mol CO₂) x (84.01 g NaHCO₃/1 mol NaHCO₃) = 0.8545 g sodium bicarbonate

   6. Calculate the student’s percent error.
      {12343_PreLabAnswers_Equation_1}

      actual = 0.8545 g sodium bicarbonate
      theoretical = (0.293 x 3.02 g) = 0.8849 g sodium bicarbonate
      % error = |0.8545 g – 0.8849 g|/0.8849 g x 100 = 3.435%

   7. Would the student’s results change, if he had decided to titrate the sample, instead of collecting the gas? Explain.

      Yes, the final amount of sodium bicarbonate would be calculated and found to have a smaller amount than what is actually included in the sample. As soon as a titration on the sodium bicarbonate is started, the other acidic components in the baking powder would also react with the sodium bicarbonate. Thus, a smaller than actual amount of titrant would be used, and a smaller than actual amount of sodium bicarbonate would be recorded.

3. A student is given a 2.00-g mixture of sodium bicarbonate, anhydrous citric acid and corn starch. The student places all 2.00 g of the mixture in a gel capsule inside an Erlenmeyer flask and sets up a gas collection apparatus like below. She then adds 10.0 mL of 2.0 M HCl to the flask and quickly places the rubber stopper over the flask. Once the reaction is complete, the student collects 6.0 x 10^–3 moles of dry carbon dioxide gas. She then titrates the leftover solution with 1.0 M NaOH and phenolphthalein as the indicator. The initial volume reading from the buret is 0.00 mL. The solution reaches its endpoint at 17.35 mL.
   {12343_PreLabAnswers_Figure_5}
   1. Write the balanced chemical reaction for citric acid and sodium bicarbonate.

      3NaHCO₃(s) + H₃C₆H₅O₇(aq) → 3H₂O(l) + 3CO₂(g) + Na₃C₆H₅O₇(aq)

   2. Write the balanced chemical reaction for hydrochloric acid and sodium bicarbonate.

      NaHCO₃(s) + HCl(aq) → H₂O(l) + CO₂(g) + NaCl(aq)

   3. What was the mass of each component in the original 2.00-g mixture? Show all work.

      Moles of CO₂ produced = moles of NaHCO₃(s) that reacted.
      6.0 x 10^–3 moles CO₂ = 6.0 x 10^–3 moles NaHCO₃(s)
      6.0 x 10^–3 moles NaHCO₃(s) x (84.01 g/mol) = 0.50406 g = 0.50 g NaHCO₃
      17.35 mL of 1.0 M NaOH = 0.01735 moles NaOH neutralized the excess acid in the solution
      10.0 mL of 2.0 M HCl = 0.020 moles of HCl initially added
      0.020 moles of HCl – 0.01735 moles NaOH = 0.00265 moles HCl
      6.0 x 10^–3 moles NaHCO₃ – 0.00265 moles HCl = 0.00335 moles NaHCO₃
      0.00335 moles NaHCO₃ x (1 mol H₃C₆H₅O₇/3 moles NaHCO₃) = 0.001117 moles H₃C₆H₅O₇
      0.001117 moles H₃C₆H₅O₇ x (192.13 g/mole) = 0.21 g of citric acid
      2.00 g – 0.50 g – 0.29 g = 1.29 g corn starch

   4. In another sample of the same mixture, after adding 10.0 mL of 2.0 M HCl, a student found the sodium bicarbonate to be approximately 0.45 g of the 3.00-g sample. When the remaining mixture was titrated, it took 23.40 mL of the 1.0 M NaOH to reach the endpoint. What was the amount of citric acid in this sample? Show all work.

      0.45 g sodium bicarbonate
      23.40 mL of 1.0 M NaOH = 0.0234 moles NaOH.
      10.0 mL of 2.0 M HCl = 0.020 moles HCl initially added
      0.0234 moles – 0.020 moles = 0.0034 moles NaOH
      0.0034 moles NaOH x (1 mole H₃C₆H₅O₇/3 moles NaOH) = 0.001133 moles H₃C₆H₅O₇
      (0.45 g sodium bicarbonate)/(84.01 g/mole) = 0.00536 moles sodium bicarbonate reacted
      0.00536 moles sodium bicarbonate x (1 mole H₃C₆H₅O₇/3 moles NaOH) = 0.001786 mol H₃C₆H₅O₇
      0.001133 moles H₃C₆H₅O₇ + 0.001786 moles H₃C₆H₅O₇ = 0.002919 moles H₃C₆H₅O₇
      0.002919 moles H₃C₆H₅O₇ x (192.13 g/mole) = 0.56 g citric acid

4. In the lab, you will have to analyze a 3.00-g mixture of potassium chloride, potassium hydroxide, and potassium carbonate. Write a step-by-step procedure to determine the percent of each chemical in the 3.00 g sample.
   1. Think safety, first. Make sure you have the proper PPE available to perform this lab (i.e., goggles, apron, gloves).
   2. Make a list of the equipment and glassware needed for this lab.
   3. Number the steps in your procedure; remember to be as detailed as possible, from setup to cleanup.
   4. Draw the necessary data tables in your notebook for data collection during the lab.
   5. Draw the setup(s) you will be using in the lab. Label all equipment.
   6. Write the reactions occurring in your procedure. Label the reaction with the step it is occurring in your procedure.

Sample Data

Sample Procedure

1. Obtain an unknown numbered capsule from the teacher. Record the unknown number in the data table.
2. Mass an empty gel capsule and the unknown filled capsule. Record both masses in the data table.
3. Place the capsule in a clean 250-mL Erlenmeyer flask.
4. Obtain a one-holed rubber stopper, rubber tubing and glass tubing.
5. Set up a gas collection system like the one drawn below. {12343_PreLabAnswers_Figure_5}
6. Fill the pneumatic trough with water. Make sure to have the hose held up so the water does not leak out of the bottom.
7. Fill a separate collection bottle completely full with water and cover with a watch glass. Place upside down in the water trough.
8. Add 10.0 mL of 6 M HCl to the flask with the capsule.

   K₂CO₃(s) + 2HCl(aq) → 2KCl(aq) + CO₂(g) + H₂O(l)
   KOH(s) + HCl(aq) → KCl(aq) + H₂O(l)

9. Immediately cap the flask with the rubber stopper/tube and begin collecting gas in the trough.
10. Wait until the reaction is no longer bubbling. If needed, you may swirl the flask.
11. When the reaction is complete, line up the water line of the collection bottle and the trough. Mark the line on the bottle with a wax pencil.
12. Fill the collection bottle with water to the line. Measure the water in a graduated cylinder and record the total volume.
13. Record the temperature and pressure in the room.
14. Calculate the moles of carbon dioxide produced.
15. Calculate the mass of potassium carbonate that was reacted.
16. Add 2–3 drops of phenolphthalein to the acid/capsule mixture.
17. Set up a buret with 1.0 M sodium hydroxide. Make sure the buret is clean and then rinse with the 1.0 M sodium hydroxide solution.
18. Record the initial volume of the buret.
19. Begin to slowly titrate the solution.

    HCl(aq) + NaOH(aq) → + H₂O(1) + NaCl(aq)

20. When the solution stays the faintest pink, record the total volume.
21. Calculate the amount of potassium hydroxide in the mixture.
22. Calculate the amount of potassium chloride in the mixture.

Data Table. Unknown Capsule 5 Reactions from Lab:
K₂CO₃(s) + 2HCl(aq) → 2KCl(aq) + H₂O(l) + CO₂(g)
KOH(s) + HCl(aq) → KCl(aq) + H₂O (l)
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

Example Calculations:
Mass of K₂CO₃ in sample:

PV = nRT
(0.9556 atm)(0.163 L) = n(0.08206 atm•L•mol^–1•K^–1)(22 + 273.15)
n = 0.00643 moles of CO₂
Moles of CO₂ = moles of K₂CO₃
Mass of potassium carbonate = 0.00643 x (138.21 g/mole) = 0.89 g K₂CO₃

Mass of KOH in sample:

Initial moles of hydrochloric acid = (0.01 L) 6M = 0.06 moles HCl
Moles of HCl reacted with potassium carbonate: 0.00643 moles K₂CO₃ x (2 moles HCl/1 mole K₂CO₃) = 0.01286 moles HCl reacted with potassium carbonate
Moles of HCl reacted with NaOH = 0.0343 mL x 1 M = 0.0343 moles NaOH = 0.0343 moles HCl
0.06 moles HCl – 0.01286 moles HCl – 0.0343 moles HCl = 0.01284 moles HCl reacted with KOH
0.01284 moles KOH x (56.11 g/mol) = 0.72 g KOH

Mass of KCl in sample:

3.15 g – 0.89 g – 0.72 g = 1.54 g KCl

References

College Board, The. 2014. “AP Chemistry Course and Exam Description, rev. ed.” NY: The College Board. Accessed June 7, 2017. http://media.collegeboard.com/digitalServices/pdf/ap/ap-chemistry-course-and-exam-description.pdf

CRC Handbook of Chemistry and Physics, 95th ed. CRC Press: Boca Raton, FL, 2014

Introduction

Most of the substances that we come in contact with every day—from the air we breathe to the water we drink—are mixtures. In this lab, you will be analyzing a mixture and identifying the percent mass of each component. An added value is that this multi-step analysis lab will give you the oppurtunity to explore concepts from multiple Big Ideas (1, 3 and 6) covered on the AP exam. Challenge yourself to see how precise you can be!

Concepts

• Mixtures
• Gases
• Equilibrium
• Acids and bases
• Stoichiometry

Background

A mixture is a combination of two or more pure substances that retain their separate chemical identities and properties. Since the amounts of each substance making up a mixture can be changed, the physical properties of a mixture depend on its composition. In contrast, the composition of a pure substance is constant, and thus pure substances have characteristic physical properties that do not change.

Mixtures are all around us. Everything from food to clothing are made of mixtures. For example, in the air we breathe, there are a mixture of gases (e.g., nitrogen, oxygen, carbon dioxide). When the individual pressures of each gas in the atmosphere are added up, the total is equal to the pressure exerted by the air. This can be displayed with Equation 1, Dalton’s law of partial pressures.

In lab, a pure gas may be collected from a chemical reaction. How the gas is collected is important. When the gas is collected over water, the vapor pressure of water must also be considered (see Equation 2). The vapor pressure of water is dependent on temperature (see Table 1). Once the pressure of the gas is determined, moles of gas can be calculated by using the ideal gas law. To do this, the volume and temperature of the gas are needed, in addition to the pressure. See Equation 3. where

P = pressure (atm)
V = volume (L)
n = number of moles
R = universal gas constant, 0.08206 L•atm•mol^–1•K^–1
T = temperature (K)

In addition to gas law analysis in this lab, acid-base neutralization and titration will be used. Titration is a method of volumetric analysis—the use of volume measurements to analyze an unknown. In acid-base chemistry, titration is most often used to analyze the amount of acid or base in a sample or solution. Consider a solution containing an unknown amount of hydrochloric acid. In a titration experiment, a known volume of the hydrochloric acid solution would be “titrated” by slowly adding dropwise a standard solution of a strong base, such as sodium hydroxide. A standard solution is one whose concentration is accurately known. The titrant, sodium hydroxide in this case, reacts with and consumes the acid via a neutralization reaction (Equation 4). The exact volume of base needed to react completely with the acid is measured. This is called the equivalence point of the titration—the point at which stoichiometric amounts of the acid and base have combined. Knowing the exact concentration and volume added of the titrant gives the number of moles of sodium hydroxide. The latter, in turn, is related by stoichiometry to the number of moles of hydrochloric acid initially present in the unknown.

M_a is the molarity of the acid and V_a is the volume of the acid.
M_b is the molarity of the base and V_b is the volume of the base.

Either acids or bases may be titrated to determine their concentration by choosing an appropriate standard solution as the titrant. Indicators are added to acid–base titrations to detect the equivalence point. The endpoint of the titration is the point at which the indicator changes color and signals that the equivalence point has indeed been reached. For example, in the case of the neutralization reaction shown in Equation 4, the pH of the solution would be acidic (< 7) before the equivalence point and basic (> 7) after the equivalence point. The pH at the equivalence point should be exactly 7, corresponding to the neutral products (sodium chloride and water). An indicator that changes color around pH 7 is therefore a suitable indicator for the titration of a strong acid with a strong base.

The progress of an acid–base titration can also be followed by measuring the pH of the solution being analyzed as a function of the volume of titrant added. A plot of the resulting data is called a pH curve or titration curve. Titration curves allow a precise determination of the equivalence point of the titration without the use of an indicator. The graph of pH versus volume of NaOH added for the titration of HCl is shown in Figure 1. Note the significant change in pH in the vicinity of the equivalence point.

Experiment Overview

The purpose of this activity is to promote conceptual and practical understanding of how to analyze a mixture. First, review and analyze a dry mixture in the Prelab Homework Assignment. After completing the homework assignment, design a procedure to identify the amounts of various chemicals in a dry inorganic mixture. Based on the individual properties of the chemicals in the mixture, different analysis techniques will be needed, such as gas collection and acid–base neutralization or titration. See how close your analysis is to the actual percentages of the mixture! You’ll love the challenge!

Prelab Questions

Complete the following homework set and write a lab procedure to be approved by your instructor prior to performing the lab. Along with your procedure, turn in any graphs, tables or figures you are asked to create in this homework set, and answers to all questions. Use a separate sheet of paper if needed.

1. The ingredients of two different brands of baking powder are shown below. Research the ingredients and correctly label which ingredients act acidic or basic when used in cooking.

   Baking Powder A’s Ingredient List: Sodium acid pyrophosphate, Sodium bicarbonate, Corn starch and Monocalcium phosphate
   Baking Powder B’s Ingredient List: Corn starch, Sodium bicarbonate, Sodium aluminum sulfate, Monocalcium phosphate

   {12343_PreLab_Table_2}
2. According to the label, a sample of baking powder A is 29.3% sodium bicarbonate. A student decides to add 9.5 mL of 6 M hydrochloric acid to a 3.02-g sample of the baking powder. He then collects the gas over water. The temperature and pressure of the room are 20.0 °C and 0.99595 atm.
   1. Write the balanced chemical equation for the reaction occurring above.
   2. The student collected the gas with the setup below. Label all the unlabeled equipment that was used in the experiment.
      {12343_PreLab_Figure_3}
   3. What gas was collected?
   4. When the reaction finished and no more gas was produced, the student marked the gas/water line on the collection glass. The teacher emphasized to align the water line in the collection glass with the water line in the trough, before marking the collection glass. Why is this important?
   5. If the student collects 251.5 mL of gas, what is the mass of sodium bicarbonate that reacted?
   6. Calculate the student’s percent error.
   7. Would the student’s results change, if he had decided to titrate the sample, instead of collecting the gas? Explain.
3. A student is given a 2.00-g mixture of sodium bicarbonate, anhydrous citric acid and corn starch. The student places all 2.00 g of the mixture in a gel capsule inside an Erlenmeyer flask and sets up a gas collection apparatus like below. She then adds 10.0 mL of 2.0 M HCl to the flask and quickly places the rubber stopper over the flask. Once the reaction is complete, the student collects 6.0 x 10^–3 moles of dry carbon dioxide gas. She then titrates the leftover solution with 1.0 M NaOH and phenolphthalein as the indicator. The initial volume reading from the buret is 0.00 mL. The solution reaches its endpoint at 17.35 mL.
   {12343_PreLabAnswers_Figure_5}
1. Write the balanced chemical reaction for citric acid and sodium bicarbonate.
2. Write the balanced chemical reaction for hydrochloric acid and sodium bicarbonate.
3. What was the mass of each component in the original 2.00-g mixture? Show all work.
4. In another sample of the same mixture, after adding 10.0 mL of 2.0 M HCl, a student found the sodium bicarbonate to be approximately 0.45 g of the 3.00-g sample. When the remaining mixture was titrated, it took 23.40 mL of the 1.0 M NaOH to reach the endpoint. What was the amount of citric acid in this sample? Show all work.
4. In the lab, you will have to analyze a 3.00-g mixture of potassium chloride, potassium hydroxide and potassium carbonate. Write a step-by-step procedure to determine the percent of each chemical in the 3.00-g sample.
   1. Think safety, first. Make sure you have the proper PPE available to perform this lab (i.e., goggles, apron, gloves).
   2. Make a list of the equipment and glassware needed for this lab.
   3. Number the steps in your procedure; remember to be as detailed as possible, from setup to cleanup.
   4. Draw the necessary data table(s) in your notebook for data collection during the lab.
   5. Draw the setup(s) you will be using in the lab. Label all equipment.
   6. Write the reactions occurring in your procedure. Label the reaction with the step it is occurring in your procedure.

Safety Precautions

Hydrochloric acid is toxic by ingestion or inhalation and is severely corrosive to skin and eyes. Phenolphthalein solutions contain alcohol and are flammable liquids; they are toxic by ingestion. Do not use near flames or other sources of ignition. Dilute sodium hydroxide solution is slightly toxic by ingestion and skin absorption and is irritating to skin and eyes. Dilute potassium hydroxide solution is a skin and body tissue irritant and slightly toxic by ingestion. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the lab.