Solutions, Electrolytes, and Conductivity Lab 8

 The goal of this experiment is to illustrate the behavior of strong, weak, and non-electrolytes in aqueous solution.  This will be achieved through the use of electrical conductivity measurements.  You will be afforded more experience in solution preparation. Purpose

Dissolution vs. Dissociation

 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 Solution Preparation

 Determine the mass of the solid needed. You will need the following values first:  Molar mass of the solid  Total volume desired  Final concentration desired Calculation:  Mass, g = concentration, mol/L x molar mass, g/mol x volume, L  Remember the precision of your glassware! Solution Preparation from Solids

 Make the solution:  Weigh out the appropriate mass of solid.  Rinse the appropriate size volumetric flask a couple of times with distilled water. Discard the water rinses as waste.  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. Solution Preparation from Solids

 Determine the volume of stock solution needed. You will need the following values first:  Concentration of stock solution (M 1 )  Desired concentration of diluted solution (M 2 )  Desired volume of diluted solution (V 2 )  Calculation:  M 1 V 1 = M 2 V 2  Remember the precision of your glassware! Solution Preparation from Liquids

 Make the solution:  Obtain the appropriate volume of stock solution using a graduated cylinder. (Always add a few mL extra.)  Rinse the appropriate size volumetric flask a couple of times with distilled water. Discard the water rinses as waste.  Place a small volume of distilled water in the volumetric flask.  Use the appropriate glassware (usually but not always a pipet) to transfer the correct volume of stock solution from the graduated cylinder to the volumetric flask. (Prep the transfer glassware as appropriate.)  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. Solution Preparation from Liquids

 Strong Electrolytes 100% dissociation and high conductivity NaCl (s)  Na + (aq) + Cl - (aq)  Weak Electrolytes partial dissociation and partial conductivity CH 3 COOH (aq) CH 3 COO - (aq) + H + (aq)  Non Electrolytes no dissociation and no conductivity C 12 H 22 O 11(s)  C 12 H 22 O 11(aq) Electrolytes

 The ability of an aqueous solution to conduct electricity is dependent on the presence of ions in solution.  Conductivity or к 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. 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. CaCl 2 Why?  NaCl vs. C 6 H 5 COOH Why? Conductivity

Procedure  Soak your conductivity probe in distilled water for 30 minutes before starting your experiment. Why? Conductivity Standard Conductivity,  S/cm Distilled Water M KCl M KCl M KCl M KCl M KCl8426.9

 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.  Calibrate your conductivity probe.  Measure the conductivities of your known solutions, distilled water, tap water, and three unknowns. Procedure

 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 Safety Concerns

All neutral solutions can go down the drain with plenty of water when you are finished. Acidic solutions need to be disposed in the acid waste container in the fume hood. Remember to clean up the balances and counters if / when you spill anything. Waste

 Lab 9 is next. Lab 9 Reminder