Honors ChemistryName

Conductivity Lab

Conductivity of Aqueous Solution

Objective: You will use the Vernier software and Conductivity Probe to test the conductivity of distilled water and that of a salt solution. Then you will apply the skills learned to investigate the following questions:

  • How is the conductivity of ionic compounds affected by the number of ions per formula unit?
  • What is the relationship between acid strength and conductivity?
  • How do the conductivities of various water-soluble molecular compounds compare?
  • How does the conductivity of NaCl(aq) vary as concentration is changed?
  • How do the conductivities of NaCl, MgCl2 and AlCl3 compare?

Background:

Dissolving is a general term for when something is placed into a solution and the substance breaks up. It will seem as if the substance is no longer present, but instead the original substance breaks into small pieces. This process would be one of dissolving and not dissociation. An example of dissociation would be when NaCl is broken up into Na+ and Cl- ions independent of each other. Dissociation is "the separation of ions that occurs when an ionic compound dissolves" (Modern Chemistry by Holt, Rinehart and Winston). Thus, dissociation is more specific than dissolving. Anything that dissociates also dissolves, but not everything that dissolves can dissociate.

Electrical conductivity is based on the flow of electrons. Metals are good conductors of electricity because they allow electrons to flow through the entire piece of material. Thus, electrons flow like a “sea of electrons” through metals. In comparison, distilled water is a very poor conductor of electricity since very little electricity flows through water. Highly ionized substances are strong electrolytes. Strong acids and salts are strong electrolytes because they completely ionize (dissociate or separate) in solution. The ions carry the electric charge through the solution thus creating an electric current. Slightly ionized substances are weak electrolytes. Weak acids and bases would be categorized as weak electrolytes because they do not completely dissociate in solution. Substances that do not conduct an electric current are called non-electrolytes. Non-electrolytes do not ionize; they do not contain moveable ions.

Representation of Species in Solution:

Ionic compounds:

When a solid ionic compound dissolves in water, the ions become separated, and are written as dissociation reactions.

Note that the equation must remain balanced, with the same number of each type of ion on either side of the arrow. Also, the charges of ions must always be included when written alone in solution, but never when part of an ionic compound. Several examples are shown below, with (aq) representing a species dissolved in water. Since ions are formed in each case, they would all conduct a current.

MgCl2 (s) ------> Mg2+ (aq) + 2 Cl– (aq)

Ba(NO3)2 (s) ------> Ba2+ (aq) + 2 NO3– (aq)

Al2(SO3)3 (s) ------> 2 Al3+ (aq) + 3 SO32– (aq)

So, for example, the proper representation of MgCl2 in solution would be as Mg2+ (aq) + 2 Cl– (aq).

Covalent Compounds:

Other than acids and bases (discussed later), the covalent compounds in this class that dissolve in water do so without

breaking any covalent bonds. These molecular species dissolve without forming any ions, and therefore will not conduct acurrent and are considered nonelectrolytes. Some examples of these species dissolving in water are shown below.

CH3CH2OH (l) ------> CH3CH2OH (aq)

C6H12O6 (s) ------> C6H12O6 (aq)

The proper representation of nonelectrolytes in solution are as their original molecular formulas in aqueous states.

Acids:

Acids are different from ionic or normal covalent compounds in that they react with water when they are dissolved

by donating a hydrogen ion (H+) to water. The new species formed are the hydronium ion (H3O+) and the anion from

that particular acid.

A general reaction for an acid, HA, is as follows, and is called an Ionization reaction:

HA (aq) + H2O (l) ------> H3O+ (aq) + A– (aq)

The extent to which the acid reacts with water determines whether it is a Strong or a Weak acid.

Strong Acids: In dilute solution, the reaction with excess water is complete (100 %).

i.e.: HCl (g) + H2O (l) ------> H3O+ (aq) + Cl– (aq)

Thus, if one of the reagents you use is any one of the following strong acids, the proper representation in solution is as thehydronium ions and anions, since these species dominate. The strong acids we will consider in are:

HCl (as H3O+ and Cl–)

HBr (as H3O+ and Br–)

HI (as H3O+ and I–)

HNO3 (as H3O+ and NO3–)

H2SO4 (as H3O+ and HSO4–)

HClO4 (H3O+ and ClO4–)

Weak Acids: These acids only react with water to a small extent (usually less than 10% completion), and so the

dominant species in solution would be the original weak acid, and would be written this way in equations. The ionization

reactions are written with reversible arrows to show that there is an equilibrium between all species in solution. An exampleis acetic acid, shown below:

HC2H3O2 (l) + H2O (l) <-> H3O+ (aq) + C2H3O2– (aq)

Note: acetic acid is sometimes written as CH3CO2H or CH3COOH, with the acidic hydrogen isolated on the far right.

Bases:

Bases are treated like acids in that there are both weak and strong bases, depending on the degree to which they react withwater. For bases, the species formed is the hydroxide ion, OH–.Strong bases ionize to 100% extent (are strong electrolytes) and are represented as the products formed. Strong bases arethe alkali metal hydroxides (NaOH, KOH, etc.) and some of the alkaline earth metal hydroxides like Ca(OH)2.Weak bases are weak electrolytes and, like weak acids, are represented as their original formulas in solution. Bases reactopposite to acids in that they accept a hydrogen ion from water. An example is ammonia:

NH3 (aq) + H2O (l) <-> NH4+ (aq) + OH– (aq)

So ammonia would be written as NH3 (aq) when used in writing equations in solution.

Safety:

Goggle, aprons and closed toe shoes are required at all times. Dispose of chemicals as directed by your instructor. Acids can cause sever burns. Wash all affected areas immediately with soap and water and inform your teacher immediately.

Materials:

Conductivity ProbeComputerLogger Mini

Distilled WaterGraduated CylinderBeakers

Stirring Rod0.01 M NaCl0.01 M MgCl2

0.01 M AlCl30.005 HCl0.005 MH3PO4

0.005 M HC2H3O20.005 M H3BO30.005 MCH3OH methanol

0.005 M C2H6O2ethylene glycol0.005 M C12H22O11Wash Bottle

Prelab:

1. What is the formula for acetic acid?

2. Define the following terms. Each definition should include what it is and why it does what it does: (See example for electrolyte).

Electrolyte – Conducts electricity readily because all of the compound dissociates into ions when dissolved in water

a)Dissolve-

b)Dissociate –

c)Nonelectrolyte –

d)Strong electrolyte –

e)Weak electrolyte –

f)Salts –

g)Ionic compounds –

h)Molecular compounds –

i)Acids-

3. To determine if a solution is conductive, a conductivity test is performed. This test is based on the an electrical circuit comprising a battery and a bulb that lights when electric current flows and therefore when the aqueous solution is conductive. The following results were found:

Tested solution / Does the bulb light ? / Current (mA)
Distilled water (almost pure) / no / 2
Tap water / no / 10
sugar water / no / 10
Salt water / yes / 220
Solution of copper sulfate / yes / 160

Based in these results answer the following questions:

a)distinguish two groups among tested solutions and summarize what observations can be made about each group.

b)An electric current consists of electrically charged particles in motion, what can be concluded about the aqueous solutions tested.

4. Write dissociation reactions for the following ionic compounds:

a) KBr (s) ------>

b) BaF2 (s) ------>

c) Fe(ClO3)2 (s) ------>

5. I, ______, read through the entire first two pages of this lab.

Part A: How do the conductivities of NaCl, MgCl2 and AlCl3 compare?

Set Up:

  1. Turn the power on for the computer.
  2. Connect the LabQuest Mini to the computer using the USB connector.
  3. Set the Conductivity Probe to the 0-2000uS/cm setting.
  4. Connect the Conductivity Probe to the LabQuest Mini utilizing channel 1.
  5. Select Logger Pro from the desk top menu.
  6. The Conductivity Probe must be rinsed into the between each test, by washing the tip gently with distilled water from your wash bottle.
  7. Select

File

Open

Chemistry with Vernier

#14 Conductivity Solutions

  1. Double click Volume (drops) column on the left side of the screen
  2. Under the Column Definition tab make the following changes:
  3. Name: Molarity
  4. Short Name: M
  5. Units: mol/L
  6. Select Done. The x-axis should now reflect these changes.

Data Collection:

  1. Using your graduated cylinder, measure 50ml of the 0.01M solution of NaCl provided by your teacher.Pour the 50 ml into a 100/150ml beaker.
  2. Place the tip of the Conductivity Probe into the solution. The hole near the tip of the probe should be completely covered by the solution.
  3. Start the data collection process by press the Green Collect button at the top right side of the menu bar.
  4. Wait a few seconds for your conductivity reading to stabilize. (Lower left corner of the screen).
  5. Select KEEP from the top right side of the menu bar. (Do note select STOP until step 16.)
  6. Type in the molarity of the solution you are testing. Select OK.
  7. Rinse the Conductivity Probe as outlined in the Set Up.
  8. Record the molarity and conductivity in data table.
  9. Now you make 5 dilutions, measuring the conductivity of each
  10. Measure 50ml of distilled water using your graduated cylinder and add this to the 50ml of your standard solution in the 100/150ml beaker. Mix well.
  11. Test the conductivity of this new solution. When the conductivity reading stabilizes, select KEEP and record the molarity of this solution. (HINT: You just performed a dilution so you will use MV=MV to determine the molarity).
  12. Record the conductivity and Molarity in data table. Show your calculations in the space provided.
  13. Rinse the Conductivity Probe as outlined in the Set Up.
  14. Using a graduated cylinder, measure 50ml of the dilution you just tested. (Hold on to the remaining solution just in case you make a mistake, otherwise you may discard it in the sink.) Add this 50ml of solution and 50ml of distilled water into the 100/150ml beaker. Mix well.
  15. Test the conductivity of this new solution and record the molarity of the new solution.
  16. Rinse the Conductivity Probe as outlined in the Set Up.
  17. Repeat steps 10 – 16 until you have successfully tested 6 concentrations.
  18. When all six concentrations have been tested, select STOP from the top right side of the menu bar.
  19. Repeat steps 1 – 18 two additional times, using first the 0.01M MgCl2 and then 0.01M AlCl3.
  20. When you attempt to collect the conductivity of the 0.01M MgCl2 solution, an ERASE DATA message will appear. Select STORE LATEST RUN. This will allow you to compare your data at a later time.
  21. Following the general procedure outlined above for conductivity testing, determine the conductivity for this solution.
  22. You will be testing the conductivity of 5 more concentrations of MgCl2 by creating ½ dilutions as outlined above.
  23. When you have completed testing the conductivity of all 6 concentrations, select STOP.
  24. Repeat steps 1 – 16 using the 0.01M AlCl3.
  25. When you attempt to collect the conductivity of the 0.01M AlCl3 solution, an ERASE DATA message will appear. Select STORE LATEST RUN. This will allow you to compare your data at a later time.
  26. When you have taken all of your data, select STOP.
  27. If you cannot see your data points, double clip either axis. Select AXIS OPTIONS.
  28. On the Y-axis select AUTO SCALE FROM 0.
  29. On the X-axis select AUTO SCALE FROM 0.
  30. To create your Best-Fit lines, Select the LINEAR FIT from the menu bar
  31. Select all three runs. OK.
  32. To add labels to your lines, select INSERT, TEXT ANNOTATION.
  33. Type in the formula of the compound tested.
  34. Point at the end of the line and drag it to touch the appropriate Best-Fit line. You can also drag the test box closer to the line.
  35. Repeat labels for all three runs.
  36. Print out your data. Select FILE, PRINT. Select the PRINT FOOTER box. Be sure to put your name in the Footer. Reports look very similar when they all print out at once to the same printer.

Data Table 1 for Part A: NaCl

Conductivity / Molarity / Molarity Calculations
0.010 M / NA

Data Table 2 for Part A: MgCl2

conductivity / Molarity / Molarity Calculations
0.010 M / NA

Data Table 3 for Part A: AlCl3

Conductivity / Molarity / Molarity Calculations
0.010 M / NA

Part B: What is the relationship between acid strength and conductivity?

Set Up:

  1. Exit out of experiment 14. Do not save your graph. (You should have already printed this in step 34 of a part A).
  2. Set the Conductivity Probe to the 0-2000uS/cm setting.
  3. Select Logger Pro from the desk top menu.
  4. The Conductivity Probe must be rinsed into the sink between each test, by washing the tip gently with distilled water from your wash bottle.
  5. Place the tip of the Conductivity Probe into 0.050M solution of HCl provided by your teacher. The hole near the tip of the probe should be completely covered by the solution.
  6. Wait a few seconds for your conductivity reading to stabilize. (Lower left corner of the screen). Record the conductivity in Data Table 1.
  7. Remove the probe from the solution.
  8. Rinse the probe tip with distilled water.
  9. Repeat steps 5 – 8 from Part B for H3PO4, HC2H3O2, and H3BO3. (Note, all of these are 0.05M solutions.)

DATA TABLE 4 for Part B.

Acid Formula / Molarity / Conductivity
HCl / 0.05 M
H3PO4 / 0.05 M
HC2H3O2 / 0.05 M
H3BO3 / 0.05 M

Part C: How do the conductivities of various water-soluble molecular compounds compare to each other and to a weak acid?

  1. Change the Conductivity Probe setting to the 0 – 200uS/cm setting.
  2. Test the conductivity using the same procedure as Part B.
  3. Record your reading in Data Table 2
  4. Repeat steps 2 – 3 for CH3OH, C2H6O2, C12H22O11, and H3BO3.
  5. All solutions may be returned to their original container after testing.

DATA TABLE 5 for Part C

Formula / Molarity / Conductivity
CH3OH / 0.05 M
C2H6O2 / 0.05 M
C12H22O11 / 0.05 M
H3BO3 / 0.05 M

Conductivity of Solution: The Effect of Concentration (74 pts)

______5ptsLab put in notebook according to directions. Includes complete sentences and

the question as part of the answer.

______10 ptsLab physically completed within one week of assigned date

______10 ptsPrelab questions completed

______2ptsData table for Part B is complete.

______2ptsData table for Part C is complete

.

______2ptsData table for Part A is completely filled out with calculations for molarity

______5ptsPart A Graph: Title and correct scale is used. Three best-fit lines are included

along with labels for each line. Do not print the data table

______2ptsPost-Lab 1: Clearly and concisely compares the slope of the three best-fit lines

for the graph in Part A.

______2ptsPost-Lab 2: Student clearly states what happens to the conductivity as the

solutions are diluted.

______8 ptsPost Lab 3: Student clearly states the relationship of concentration to

conductivity for each substance and explains this relationship at a molecular

level using mole ratios of ions present.

______6ptsPost-Lab 4: Student writes dissociation equations for all ionic compounds and

acids that dissociate.

______8ptsPost-Lab 5: Student clearly refers to the graph from Part A to note which slope is

the steepest and which slope is the slightest. Student explains in terms of dissociation and moles of ions present.

______8ptsPost-Lab 6:Using your data from part B, explain why HCl is considered a strong acid where as H3BO3 is considered a weak acid in terms of dissociation and moles of ion present.

______4 pts Post-lab 7: Using your data from part C, explain what you observed.