7.3 Activity: Investigating Electrical Conductivity

Question

Which types of solutions are good electrical conductors?

Background

In order for a solution to conduct electricity, it must contain mobile ions. The more ions there are in solution, the stronger the electrolyte.

Procedure

1 . A student has a light bulb conductivity apparatus and 100.0 mL of each of the following solutions in a beaker:

• 4.0 M C12H22O11

• 1.0 M NaCl

• 0.8 M Na2CO3

• 2.0 M CH3COOH

• 3.0 M C2H5OH (ethanol, an alcohol)

• 0.8 M HCl

• 2.0 M NH3

Make a sketch of each solution containing a conductivity apparatus. Label each beaker’s contents. Use a colored pencil to show the relative brightness of the light bulb in each.

2. Compare your answers to those of another group. Did you agree?

Results and Discussion

1. Which solutions were ionic? Covalent?

2. For each ionic solution, write a dissociation equation for the solute, and calculate the concentration of each ion present.

3. Calculate the total ion concentration of each ionic solution by adding all of the ion concentrations together. Write this value under its corresponding diagram.

4. Rank the solutions in order from highest conductivity to lowest conductivity. List the nonelectrolytes last. Use an equal sign (=) for solutions that are about equally conductive.

5. Summarize the rules for determining if a solute will make a solution that is a good electrical conductor.

7.3 Review Questions

1. Calculate the concentration of the following solutions when:

(a) 175 mL of 0.55 M HCl is added to 25 mL of water

(b) 45.0 mL of 0.035 M Na2Cr2O7 is diluted to 100.0 mL

(c) a 100.0 mL sample of 2.0 M NaOH is evaporated until the resulting volume of solution is 75.0 mL

2. Hydrochloric acid can be purchased as a 6.0 M solution. What volume of this stock solution must be used to prepare 250.0 mL of 2.5 M HCl?

3. Explain how the terms “solvation” and “hydration” are different.

4. Explain why the cation in a solute will be attracted to a solvent water molecule.

5. The solute KI is dissolved in water. Sketch a diagram showing the ions present in this solute surrounded by water molecules. Assume six water molecules surround each ion. Be sure to draw the water molecules aligned appropriately around the ion with respect to dipoles.

6. Write balanced dissociation equations to represent the dissolving of the following solutes in water:

(a) FeCl3

(b) MnHPO4

(c) Zn(SCN)2

(d) Al2(Cr2O7)3

(e) silver oxalate

(f) iron(III) sulfite

(g) chromium(II) chromate

(h) ammonium hydrogen oxalate

7. A student adds 250.0 mL of 0.60M HCl to 300.0 mL of 1.0 M HCl. What is the final [HCl]?

8. Calculate the concentration of each ion in the following:

(a) 0.20 M CuCl2

(b) 1.5 M Li2C2O4

(c) 6.0 M nitric acid

(d) 1.4 × 10–3 M magnesium permanganate

(e) the resulting solution when 18.5 mL of 0.25 M Al(NO3)3 is mixed with 22.4 mL of 0.50 M Cu(NO3)2

9. A solution is made by dissolving some solid (NH4)2CrO4 in water. If the [NH4+] = 0.60 M, what is the [CrO42–]?

10. Sketch a light bulb apparatus in a solution of KNO3. Show which ions move to the positive and negative electrodes. Is KNO3 a strong electrolyte? Explain.

11. List the acids that are strong electrolytes. Explain why they are considered strong.

12. You are given two beakers, one containing 3.0 M HNO3 and one containing 6.0 M C12H22O11. Explain how you could use a light bulb apparatus to distinguish between the two solutions.

13. Classify the following as being strong electrolyte, weak electrolyte, or nonelectrolyte, solutions.

(a) 0.1 M HClO4

(b) distilled H2O

(c) 1.0 M CH3COOH

(d) 0.1 M ZnSO4

(e) C8H18(l)

(f) 6.0 M NH3

14. Consider the following statement: “If a solution has a high concentration, it will conduct electricity well.” Do you agree or disagree? Explain.