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Solutions, Electrolytes, and Conductivity

Solutions, Electrolytes, and Conductivity. Lab 9. Outline. Purpose Solutions Solution Preparation from Solids Solution Preparation from Liquids (dilution) Electrolytes Conductivity Procedure Waste Skill Evaluation Reminder. Purpose.

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Solutions, Electrolytes, and Conductivity

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  1. Solutions, Electrolytes, and Conductivity Lab 9

  2. Outline • Purpose • Solutions • Solution Preparation from Solids • Solution Preparation from Liquids (dilution) • Electrolytes • Conductivity • Procedure • Waste • Skill Evaluation Reminder

  3. Purpose • To illustrate the behavior of strong, weak, and non-electrolytes in aqueous solution. • Achieved through the use of electrical conductivity measurements. • Solution preparation practice.

  4. Dissolution vs. Dissociation

  5. Solutions • Types of solutions we are dealing with today: • solid to liquid • liquid to liquid

  6. Solution Preparation • When you are required to make a solution of accurate concentration, a volumetric flask is used. • We never make solutions of accurate concentration in: • Beakers • Graduated cylinders • Erlenmeyer flasks

  7. Solution Preparation from Solids • Determine the mass of the solid needed by using: • Molar mass of the solid • Total volume desired • Final concentration desired • Calculation: • Mass, g = [ ], mol/L x MW, g/mol x Vol, L • Remember the tolerances on your glassware!

  8. Solution Preparation from Solids • Make the solution: • Weigh out the appropriate mass of solid. • Place a small volume of distilled water in the volumetric flask. • Add the solid to the volumetric flask. • Add some more distilled water to the flask, stopper, and invert several times. • Add distilled water to the calibration line (fill to volume) using a medicine dropper, stopper, and invert several times.

  9. Solution Preparation from Liquids • Determine the volume of stock solution needed by using: • Concentration of stock solution (M1) • Desired concentration of diluted solution (M2) • Desired volume of diluted solution (V2) • Calculation: • M1V1 = M2V2 • Remember the tolerances of your glassware!

  10. Solution Preparation from Liquids • Make the solution: • Obtain the appropriate volume of stock solution using a graduated cylinder. (Always add a few mL extra.) • Place a small volume of distilled water in a volumetric flask. • Use the appropriate pipet to transfer the correct volume of stock solution from the graduated cylinder to the volumetric flask. • Add some more distilled water to the flask, stopper, and invert several times. • Add distilled water to the calibration line (fill to volume) using a medicine dropper, stopper, and invert several times.

  11. Electrolytes • Strong Electrolytes 100% dissociation and high conductivity NaCl(s) Na+(aq) + Cl-(aq) • Weak Electrolytes partial dissociation and partial conductivity CH3COOH(aq)CH3COO-(aq) + H+(aq) • Non Electrolytes no dissociation and no conductivity C12H22O11(s) C12H22O11(aq)

  12. Conductivity • The ability of an aqueous solution to conduct electricity is dependent on the presence of ions in solution. • Conductivity or K has units of S/cm, mS/cm, or S/cm. • We measure conductivity so we can make a comparison regarding relative numbers of ions present in solution.

  13. Conductivity • The extent to which a solution conducts electricity is dependent on the proportional amount of ions present in solution. • Which of the following will have a higher conductivity? • NaCl vs. CaCl2 Why? • NaCl vs. C6H5COOH Why?

  14. Procedure • Soak your conductivity probe in distilled water for 30 minutes before starting your experiment. Why? • Calibration

  15. Procedure • Any glassware that will be containing non-electrolytes or weak electrolytes need to be rinsed thoroughly with distilled water prior to use!!! • Make up your three known solutions. • Measure the conductivity of your known solutions, distilled water, tap water, and three unknowns. • The unknowns are already at the required concentration. No dilution is necessary!

  16. Safety Concerns • Reagents: • Acetic Acid (3%) • KCl • NaCl • Sucrose • Eye Contact: • Irritation, redness, pain, and possible damage • Skin Contact: • Irritation. May cause sensitization and / or allergic reaction. Absorption may cause symptoms similar to ingestion • Inhalation: • Irritation and coughing • Ingestion: • Gastrointestinal irritation, nausea, vomiting, diarrhea, prostration, dehydration and congestion of internal organs, and violent inflammatory reactions in the gastrointestinal tract

  17. Waste All neutral solutions can go down the drain with plenty of water when you are finished. Solutions with a pH above 8 or below 6 need to go in the acid / base waste container in the fume hood.

  18. Next Week - Skill Evaluations • Read through pages 285 – 304. • Remember to bring your goggles and lab manual.

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