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LAB 6: INTRODUCTION TO SOLUTIONS- SOLUBILITY AND CONDUCTIVITY

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    PURPOSE

    The purpose of this experiment is to:

    • Explore the conductivity of various solutions.

    • Predict whether a substance acts as a strong electrolyte, weak electrolyte, or nonelectrolyte.

    INTRODUCTION

    When substances dissolve in water, they can act as strong electrolytes, weak electrolytes, or nonelectrolytes. Strong electrolytes completely ionize in water. Examples include soluble ionic compounds and strong acids (HCl, HBr, HI, HNO3, H2SO4, HClO4, HClO3). Consider the dissolution of sodium chloride, as shown below:

    NaCl (aq) → Na+ (aq) + Cl- (aq)

    When NaCl dissolves in water, it completely breaks apart into its constituent cation (Na+) and anion (Cl-). Similarly, when HCl forms a solution with water, we assume that 100% of the molecule will ionize as follows:

    HCl (aq) → H+ (aq) + Cl- (aq)

    Weak electrolytes include weak acids (any acid other than the seven listed above) and weak bases. These substances only partially ionize in water. For example, when acetic acid, HC2H3O2, dissolves in water, some of the molecules will ionize into H+ and C2H3O2-, but most of what is present in aqueous solution will remain as HC2H3O2.

    Nonelectrolytes will not ionize in water and will remain entirely as the molecule. Examples include soluble covalent compounds. When sugar, C12H22O11, is added to water, it will exist as C12H22O11 and will not break apart.

    The ions present in the solution allow an electrolyte to conduct electricity. The greater the number of ions present in the solution, the greater the conductivity. Strong electrolytes are better conductors than weak electrolytes since they have more ions present in the solution. Nonelectrolytes will not act as conductors because they do not ionize.

    One way to determine how a substance dissolves in water is by measuring the conductivity of the solution that forms. Some conductivity meters show numerical values. The larger the value, the greater the conductivity. Others use a light to indicate conductivity. The brighter the light, the greater the conductivity. In this experiment, you will use battery-operated conductivity meters to measure the conductivity of 12 different solutions/substances. A bright green light means strong conductivity and a red light indicates no conductivity. Based on the light produced, you will then infer whether the solution acts as a strong electrolyte, weak electrolyte, or nonelectrolyte.

    SAFETY PRECAUTIONS

    1) Always wear chemical splash goggles while working with chemicals in this experiment.

    2) Use the conductivity meters carefully and rinse the electrodes after each use in each solution and when you are finished with the experiment.

    3) Dispose of all waste, as directed by your instructor, and thoroughly clean your work area and all equipment used.

    EQUIPMENT* AND CHEMICALS NEEDED

    • Well plate

    • Disposable plastic droppers

    • Deionized water

    • Ethanol

    • Gatorade

    • Tap water

    • 0.5 M - 1 M solutions of NaCl, CaCl2, HCl, HC2H3O2, NaOH, NH3, NaHCO3, C12H22O11 (Be sure the concentration of each solution is the same.)

    • Battery-operated conductivity meter with light bulb

    * Images of equipment needed in this lab can be found in the appendix (the equipment may differ slightly or be subject to changes or updates; follow your instructors’ directions).

    EXPERIMENTAL PROCEDURE

    1) Obtain a well plate and half-fill the wells with the following solutions: NaCl, CaCl2, HCl, HC2H3O2, NaOH, NH3, NaHCO3, C12H22O11, ethanol, Gatorade, tap water, and deionized water. To do this, you could use the droppers from the bottles in which the solutions are placed or disposable plastic droppers.

    2) Test the conductivity of each solution using the conductivity meter. Record the color of light produced in the data table. Be sure to rinse the electrodes on the conductivity meter with deionized water between each solution used.

    3) Pour the contents of the well plate into the sink and wash with plenty of soap and water.

    4) Thoroughly clean your work area and all equipment used. Return equipment to the appropriate place before leaving the lab.

    PRE-LAB QUESTIONS Name ____________________________________

    1) In your own words, define the following terms:

    Strong Acid:

    Weak Acid:

    Strong Base:

    Weak Base:

    Ionic Compound:

    Covalent Compound:

    Solvent:

    Solute:

    Solution:

    2) Classify the following compounds as ionic or covalent.

    Ca(NO3)2 :

    AlCl3 :

    C6H12O6 :

    NH3 :

    3) Classify the following compounds as Strong Acid, Weak Acid, Strong Base, or Weak Base.

    HBr :

    H2SO4 :

    NH3 :

    KOH :

    HF :

    4) When a solution is tested with a battery-operated conductivity meter, a bright green light is shown. Based on this observation, would you predict the solution to act as a strong electrolyte, a weak electrolyte, or a nonelectrolyte?

    DATA AND CALCULATIONS

    Under the heading “Results of Conductivity Test,” describe the color of the light bulb. For classification of solution, write strong electrolyte, weak electrolyte, or nonelectrolyte.

    Solution

    Results of the Conductivity Test

    Classification of Solution

    NaCl

    CaCl2

    HCl

    HC2H3O2

    NaOH

    NH3

    NaHCO3

    C12H22O11

    Ethanol

    Gatorade

    Tap water

    Deionized water

    POST-LAB QUESTIONS

    1) Summarize the results of the experiment concerning the color of the lightbulb turned in the presence of different types of solutions.

    2) Account for differences in the results of the conductivity test on tap water and deionized water.

    3) Vinegar is made up of 5% acetic acid (HC2H3O2) in water. Based on the results of this experiment, what would you expect to see if vinegar is tested with the conductivity meter?

    4) Explain the results of the conductivity test on Gatorade.

    5) Which solution would you expect to have a higher conductivity: NaCl or CaCl2? Explain.

    This page titled LAB 6: INTRODUCTION TO SOLUTIONS- SOLUBILITY AND CONDUCTIVITY is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Saadia Khan.