EXPERIMENT 20. Solutions INTRODUCTION

Transcription

1 EXPERIMENT 20 Solutions INTRODUCTION A solution is a homogeneous mixture. The solvent is the dissolving substance, while the solute is the dissolved substance. A saturated solution is one in which the maximum amount of solute is dissolved in a given amount of solvent, at a particular temperature. An unsaturated solution is one in which less than the maximum amount of solute is dissolved in a given amount of solvent, at a particular temperature. A supersaturated solution is one in which there is excess solute dissolved in a given amount of solvent for a particular temperature. The phrase likes dissolve likes describes the general principle of solubility: ionic and polar solutes dissolve in polar solvents, and nonpolar solutes dissolve in nonpolar solvents. Water is a polar solvent. It therefore dissolves ionic compounds such as table salt, NaCl, and polar compounds such as table sugar, C 12 H 22 O 11. Hexane is a nonpolar solvent, and dissolves nonpolar solutes. Hexane is therefore not a solvent for table salt or table sugar. Nonpolar solvents do not dissolve ionic or polar compounds. Liquids that dissolve in one another are said to be miscible. Once again, the general principle of likes dissolve likes dictates whether two liquids are miscible. If two liquids are both polar, they are miscible. If two liquids are nonpolar, they are miscible also. However, a polar liquid and a nonpolar liquid are immiscible. This means they do not dissolve in each other, and actually separate into two layers. 2 Miscible liquids 2 Immiscible liquids The amount of dissolved solute in a given amount of solvent is termed concentration. Dilute and concentrated are words used to describe qualitatively a solution s concentration. To describe quantitatively a solution s concentration, the numerical amount of solute is given per the numerical amount of either the solution or solvent. Molarity is a way of expressing quantitatively a solution s concentration. Molarity gives the quantity of solute dissolved (measured in moles) per volume of solution (measured in liters). A standard solution is a solution of known concentration, or known molarity. You will calculate how many grams of a solute are needed to prepare a specific volume of a standard solution, and then you will prepare the solution. 137

2 PROCEDURE 1. Except for the laboratory handout, remove all books, purses, and such items from the laboratory bench top, and placed them in the storage area by the front door. For laboratory experiments you should be wearing closed-toe shoes. Tie back long hair, and do not wear long, dangling jewelry or clothes with loose and baggy sleeves. Open you lab locker. Put on your safety goggles, your lab coat, and gloves. PART A SUPERSATURATION (DEMONSTRATION) 2. Add crystals of sodium acetate trihydrate, NaC 2 H 3 O 2. 3H 2 O, into a test tube until it is one-quarter full. Add just enough deionized water to cover the crystals. Place the test tube in a hot water bath to dissolve all of the crystals, then remove the test tube from the hot water bath and allow the solution to cool undisturbed. 3. After the test tube has cooled, show that the solution is supersaturated by dropping a tiny crystal of sodium acetate trihydrate into the solution. Observe the results and record your observation in your Data Table. PART B - SOLUBILITY 4. Place four small, dry test tubes in your test tube rack labeled A through D. Pour a couple of crystals of potassium permanganate into your scoopula. Add one small crystal of potassium permanganate to test tubes A and B. Add 2 ml of deionized water to test tube A, and 2 ml of hexane to test tube B. Set a vortex mixer to auto and to a speed setting of 5. Press one of the test tubes into the rubber cup, allowing the contents of the test tube to be mixed for 30 seconds. Do the same for the other test tube. Observe and record in your Data Table whether the crystal was soluble (S) or insoluble (I) in each solvent. 5. Take your scoopula to Fume Hood B, and pour a couple of crystals of iodine into the scoopula. Add one crystal of iodine to test tubes C and D. Add 2 ml of deionized water to test tube C, and 2 ml of hexane to test tube D. Mix each for 30 seconds with a vortex mixer. Observe and record in your Data Table whether the crystal was soluble (S) or insoluble (I) in each solvent. Save all solutions in a 400- ml waste beaker at your lab bench. At the end of Parts B and C, the waste beaker can be emptied into the Liquid Waste bottle in Fume Hood A. PART C - MISCIBILITY 6. Place four dry test tubes in your test tube rack labeled E through H. Place 2 ml of methanol into test tubes E and F. Add 2 ml of deionized water to test tube E, and 2 ml of hexane to test tube F. Mix each for 30 seconds with a vortex mixer. Observe and record in your Data Table whether the liquids were miscible (M) or immiscible (I). 7. Place 2 ml of heptane into test tubes G and H. Add 2 ml of deionized water to test tube G, and 2 ml of hexane to test tube H. Mix each for 30 seconds with a vortex mixer. Observe and record in your Data Table whether the liquids were miscible (M) or immiscible (I). Save all solutions in a waste beaker at your lab bench. At the end of Parts B and C, the waste beaker can be emptied into the Liquid Waste bottle in Fume Hood A. 138

3 PART D PREPARATION OF A STANDARD SOLUTION 8. Obtain a volumetric flask and a snap cap from the back counter. Obtain a solid reagent and a specific solution to prepare from the instructor. In your Data Table record the reagent, molarity, and the volume of solution you are to prepare. 9. Calculate the mass of the reagent needed to prepare your specific solution. Bring your lab report and the reagent bottle to the instructor to check your calculation. The instructor must record in question 1 that you have performed this calculation correctly before you can prepare the solution. 10. Fill the volumetric flask one-quarter full with deionized water. Attach the snap cap to the top of the volumetric flask and invert once. If there are any drops on the inside of the neck of the flask, you must get a test tube brush and soap, and scrub the inside of the neck. Careful, don t break it! Rinse the flask out completely with deionized water, fill it one-quarter full of deionized water again, replace the stopper, and invert. If no drops appear on the inside of the neck, you may continue. 11. Bring a clean, dry 100-mL beaker and your microspatula to a milligram balance. Place the beaker on the milligram balance and tare it. 12. Transfer a small amount of your reagent from its reagent bottle to a weighing cup or a glass or porcelain container by pouring. CAUTION: Never place your microspatula or scoopula into a reagent bottle. Remove the beaker from the balance chamber. Add the required amount of solid reagent to the beaker, place it back on the balance, and read the exact mass. In your Data Table, record the name of the reagent and the mass of the reagent weighed out. CAUTION: Never pour solid or liquid reagents back into stock bottles. Any excess chemicals should be properly discarded in the waste bottle in Fume Hood A. 13. Add about 30 ml of deionized water to the solid in the beaker, and dissolve the solid. If more deionized water is needed to dissolve the solid, add more. If your solid does not dissolve in 50 ml of deionized water, consult your instructor. 139

4 14. Support a glass funnel in a clay triangle on an iron ring, attached to a ring stand. Place the volumetric flask below, and adjust the height of the funnel so that the stem of the funnel is inside the neck of the flask. 15. Pour the solution into your volumetric flask through the funnel. Rinse the beaker with 5-10 ml of deionized water, and pour this through the funnel into the volumetric flask. Rinse the beaker again with 5-10 ml of deionized water, and pour this through the funnel into the volumetric flask. 16. Carefully fill the flask with deionized water until the bottom of the meniscus is about 1 cm below the hairline. Remove the volumetric flask from under the funnel, and using your medicine dropper, add the final amount of deionized water so that the bottom of the meniscus sits right on the hairline. Attach the snap cap and invert the flask 25 times to insure complete mixing. Bring your solid reagent and your solution to the instructor. 17. Rinse the volumetric flask several times with deionized water. Wipe the outside dry, and return it and the snap cap to the the back counter. 18. Clean and wipe dry your laboratory work area and all apparatus. When you have completed your lab report have the instructor inspect your working area. Once your working area has been checked your lab report can then be turned in to the instructor. 140

5 EXPERIMENT 20 LAB REPORT Name: Student Lab Score: Date: Lab Station Number: DATA TABLE PART A OBSERVATIONS PART B Water (H 2 O) Hexane (C 6 H 14 ) Potassium permanganate (KMnO 4 ) Iodine (I 2 ) PART C Water (H 2 O) Hexane (C 6 H 14 ) Methanol (CH 3 OH) Heptane (C 7 H 16 ) Molarity of Solution. M Volume of Solution. ml 1 Mass of. g 141

6 CALCULATIONS

7 POSTLAB QUESTIONS 1. Have the instructor check off the calculation for your solution. 2. Referring to your observations in Part B of the Data Table, predict whether the following crystals are ionic (I) or nonpolar (N): Potassium permanganate Iodine 3. Complete the following general solubility table. Indicate whether each solute across the top is soluble (S) or insoluble (I) with each solvent along left side. Ionic Solid Polar Solid Nonpolar Solid Polar Liquid Nonpolar Liquid 4. Referring to your observations in Part C of the Data Table, predict whether the following liquids are polar (P) or nonpolar (N): Methanol Heptane 5. Complete the following general solubility table. Indicate whether each solute across the top is miscible (M) or immiscible (I) with each solvent along left side. Polar Liquid Nonpolar Liquid Polar Liquid Nonpolar Liquid 6. Predict whether the rate of solubility increases (I) or decreases (D) for each of the following: Heating the solution Stirring the solution Grinding the solute 143

8 7. If 16.4 g of zinc bromide are dissolved in enough water to make ml of solution, what will be the molarity of the solution? Box your answer. 8. How many grams of magnesium nitrate are needed to produce ml of a M solution? Box your answer. 144