Notes on Preparing a Chemical Solution

Introduction

• Lecture by Jared Heyman, Assistant Professor of Chemistry, Elon University.
• Focus on preparing a chemical solution of a specific concentration.

Key Definitions

• Solution: A homogeneous mixture of two or more chemicals.
  • Solute: The smaller amount of a substance in the solution (e.g., sodium chloride).
  • Solvent: The larger amount of a substance in the solution (e.g., water).

Safety Considerations

• Always follow proper lab safety procedures.
• Be aware of chemical hazards associated with the substances used.

Stoichiometry and Moles

• Mole: A counting term equivalent to 6.022 x 10²³ entities.
  • Example: 1 mole of carbon has a mass of 12.01 g.
• Sodium Chloride Composition:
  • 1 mole of sodium (22.98 g) + 1 mole of chlorine (35.45 g) = 58.43 g/mole.

Concentration Measurement

• Molarity (M): Indicates moles of solute per liter of solution.
  • Expressed as moles/Liter.
• Important to measure the volume of the solution, not just the solvent.

Preparing a 0.25 M Sodium Chloride Solution

• Target Preparation: 500 mL of 0.25 M sodium chloride solution.
• Calculate the required moles:
  • 0.25 moles/L x 0.5 L = 0.125 moles of sodium chloride.
• Determine mass needed:
  • 0.125 moles x 58.43 g/mole = 7.34 g of sodium chloride.

Lab Procedure

1. Measuring Sodium Chloride:

   • Use clean lab scoop and balance with weighing paper.
   • Measure as accurately as possible (7.34 g).
   • Do not return excess reagent to the original container.

2. Transferring Solute:

   • Use a volumetric flask for accurate measurement.
   • Transfer solid sodium chloride to a 500 mL volumetric flask.
   • Use a spatula or water to ensure all solute is transferred.

3. Dissolving the Solute:

   • Add water until the flask is half full to allow for swirling.
   • Ensure all solute is fully dissolved before proceeding.

4. Filling to Volume:

   • Carefully fill the flask to the etched line using water.
   • Add water dropwise as you approach the line, watch for the meniscus.
   • The bottom of the meniscus should be level with the line.
   • Verify that the solution volume is exactly 500 mL.

5. Calculating Exact Concentration:

   • Use mass measured (7.34 g) and formula mass (58.43 g/mole):
     • Total moles = 7.34 g / 58.43 g/mole = 0.1253 moles.
   • Molarity = 0.1253 moles / 0.5 L = 0.251 M sodium chloride.

6. Labeling Solution:

   • Clearly label the storage container with the concentration and contents.

Conclusion

• Successful preparation of a sodium chloride solution from solid.
• Reference Carolina for lab supplies and assistance with chemistry demonstrations.