Solution Stoichiometry Quantitative Preparation of Aqueous Solutions

Transcription

1 Solution Stoichiometry Quantitative Preparation of Aqueous Solutions Introduction Page 1 of 11 In this week s lab, you and your lab partner will apply your prelab calculations to the preparation of aqueous solutions of known concentrations. The objectives of the lab include: calculation of solution concentrations based on the mass and molecular weight of solutes calculation of solution concentrations based on the dilution of stock solutions preparation of these solutions testing of your solution concentrations based on density and acid/base neutralization reactions observation of a rather unique example of aqueous chemistry Part I. Preparation of Solutions Using the calculations completed in the pre-lab, prepare solutions B and C exactly as outlined below. Be sure to read the mass of reagents to three decimal places when using the electronic balances. Solution A. 3.6 M H 2 O 2 solution already prepared in hood. (Caution!! Use gloves when handling!) Solution B. Outlined in the steps below, you will first make 100 ml solution of 0.08 M H 2 SO 4 from the 1.00 M stock. You will then make 50 ml solution of 0.20 M potassium iodate (KIO 3 ) using 50 ml of your 0.08 M H 2 SO 4 solution just prepared, as the solvent. The remaining 50 ml of 0.08 M H 2 SO 4 solution will be used for titration on Part II C. Step M H 2 SO 4 Using a graduated cylinder, add ml of 1.00 M H 2 SO 4 into a 100 ml volumetric flask. Rinse the graduated cylinder once with about 15 ml of distilled water, and add this rinse to the same volumetric flask. Now, fill your volumetric flask to the 100 ml mark with distilled water. Swirl this solution thoroughly to mix. Label as 0.08 M H 2 SO 4 Step M KIO 3 in 0.08 M H 2 SO 4. Add grams of potassium iodate (KIO 3 ) to a 50 ml volumetric flask and then add 0.08 M H 2 SO 4, from step 1 above, up to the 50 ml mark.. You may need to swirl or stir this solution to dissolve the KIO 3. Label this Solution B and set aside. Note: You still have 50 ml of 0.08 M H 2 SO 4 left over to be used in titration!

2 Page 2 of 11 Solution C. You need to make 50 ml of a combined solution containing 0.15 M malonic acid (C 3 H 4 O 4 ), and 0.02 M manganese sulfate monohydrate (MnSO 4 H 2 O). Add grams of malonic acid (C 3 H 4 O 4 ) and grams of manganese sulfate monohydrate (MnSO 4 H 2 O) to a 50 ml volumetric flask and fill to the 50 ml mark with distilled water. Swirl to dissolve. Label this Solution C and set aside.

3 Page 3 of 11 NAME: PARTNER: DATE: SECTION: Data Sheet: Solutions Part II. Testing the concentration of your solutions. You will be graded on the accuracy of your solution concentrations based on the results obtained below. A) Density of solution C Obtain a clean, dry 50 ml graduated cylinder, and determine its mass to three decimal places. Fill the graduated cylinder with approximately 40 ml of your Solution C, record the exact volume, and determine the mass of the filled graduated cylinder to three decimal places. Data and Calculations (use correct units!). Mass of empty graduated cylinder Volume of Solution C added to graduated cylinder Mass of graduated cylinder containing Solution C Mass of Solution C Calculated density of Solution C B) Stoichiometry of an acid/base neutralization reaction. Complete the following equation by writing the Molecular, Ionic, and Net Ionic Equations: (i) Balance the following complete molecular equation for the neutralization of sulfuric acid (Molecular) H 2 SO 4 + NaOH H 2 O + Na 2 SO 4

4 Page 4 of 11 (ii) Write the balanced complete ionic equation for this reaction and identify the spectator ions. (Ionic equation) (iii) Write net ionic equation for this reaction. (Net Ionic) C. Simple Titration of 0.08 M H 2 SO 4. Obtain approximately 10 ml of 0.08 M NaOH in a clean, dry beaker or flask. Using an eye dropper add drops of your 0.08 M H 2 SO 4 (solution B, step 1) to a clean, dry large test tube and record the exact number of drops you added. Add one drop of phenolphthalein indicator to the test tube. The indicator should be colorless in the acid solution. Use a similar dropper to add 0.08 M NaOH solution to the same test tube, counting the drops as you go, and swirling the test tube to mix the solutions. Keep adding the NaOH dropwise until the solution is just barely pink. You now have a neutral solution. If the solution is deep pink or red, discard this sample and start over. Record the number of drops of NaOH solution added. Repeat this titration two more times. Show calculations. Trial # drops of 0.08 M H 2 SO 4 used drops of 0.08 M NaOH used ml of 0.08 M H 2 SO 4 used*** ml of 0.08 M NaOH used*** moles of H 2 SO 4 used moles of NaOH used average moles of H 2 SO 4 average moles of NaOH ***Assume that one drop = ml***

5 Page 5 of 11 Determine the experimental NaOH : H 2 SO 4 mole ratio and compare this to the NaOH : H 2 SO 4 stoichiometry of your balanced molecular equation. Do they match or not? Explain. D. An unusual example of Solution Stoichiometry. Here s the big pay-off. In a clean 250 ml Erlenmeyer flask mix 10 ml each (use a graduated cylinder) of Solutions B and C, 5 drops of the Starch Solution and 2 ml 1.0 M H 2 SO 4. Now add 10 ml of Solution A (3.6M H 2 O 2 solution made up for you), swirl to mix, then place it on a piece of white paper and record all observations. CAUTION: Hydrogen peroxide is a strong oxidizing agent that can cause burns. Should you get any on your hands, immediately rinse with water. Count the total number of color cycles you observe over a 15 minute time period (colorless to blue):

6 Page 6 of 11 NAME: DATE: SECTION: Post-lab Questions: Solutions Describe how you would prepare 0.05 L of a 3.5 M solution of Na 2 S (aq) (mol. wt. of Na 2 S = g/mol). The following materials and chemicals are available for your use: A balance Pipettes 10 ml, 25 ml, 50 ml, 100 ml, 500 ml and 1L Volumetric Flasks 1 L of 5 M Na 2 S (aq) stock solution 1 L of 5 M Na 2 SO 4 (aq) stock solution 1 L of 5 M Pb(NO 3 ) 2 (aq) stock solution Distilled water 100 g of Na 2 S (s) 100 g of NaCl (s) 100 g of PbS (s) Be specific in describing the amounts and the procedures that you will use.

7 Page 7 of 11 NAME: DATE: SECTION: Pre-lab Assignment: Solutions These answers will be used in the laboratory so you must be careful about your answers. Part I. Definitions. Define each of the following terms in the space given below. Be Concise! (a) solution (b) percent concentration (c) solubility (d) precipitate (e) molarity

8 Page 8 of 11 Part II. Calculations. The following calculations must be completed before you begin this experiment. You will use the results of these calculations to guide you through the preparation of solutions in the laboratory. SHOW ALL CALCULATION WORK! Be careful of your significant figures, and be sure to correctly label all units. Solution A. This solution has already been made up for you, but you should work through the calculations for practice. You are required to make up 25.0 ml of a 3.6 M solution of hydrogen peroxide (H 2 O 2 ). You have at your disposal a stock solution that is 30% by weight, H 2 O 2, and has a density of 1.11 g/ml. Hint: A 30% by weight solution means that 100 g of the solution contains 30 g of H 2 O 2. Use this information with the density to determine the Molar concentration of 30% H 2 O 2, then apply your dilution equation to determine how you would make up 25.0 ml of 3.6 M H 2 O g of H 2 O 2 solution contains 30 g of H 2 O 2. Use the density of 1.11 g/ml to calculate the volume (ml) of 100 g of H 2 O 2 solution. Answer 2. Calculate the molecular mass of H 2 O How many moles is 30.0 g of H 2 O 2?

9 Page 9 of Based on your answers to 1 and 3, calculate the molar concentration of 30% H 2 O Based on your answer to 4, determine the volume of 30% H 2 O 2 needed to make 25.0 ml of 3.60 M H 2 O 2. Solution B. Make up a solution of 0.20 M potassium iodate (KIO 3 ) in 0.08 M sulfuric acid (H 2 SO 4 ). You have available a 1.00 M stock solution of H 2 SO You will need to make up 100 ml of 0.08 M H 2 SO 4 by dilution of a 1.00 M stock. Determine the volume (ml) of 1.00 M H 2 SO 4 needed to complete the dilution. Volume of 1.00 M H 2 SO 4 (enter this value on step 1 under Solution B on page 1) 2. What is the molecular mass of KIO 3? Answer

10 Page 10 of Calculate how many moles of KIO 3 are needed to make 50 ml of 0.20 M solution. Answer 4. Calculate how many grams of KIO 3 are needed to make 50 ml of 0.20 M solution. (enter this value on step 2 under Solution B on page 1) Solution C. Make up a 50 ml solution containing both 0.15 M malonic acid (C 3 H 4 O 4 ), and 0.02 M manganese sulfate (MnSO 4 H 2 O, the means that the compound exists as a hydrate with a molecule of water trapped in the solid. You must include it in your calculation of molecular weight). 1. What is the molecular mass of malonic acid (C 3 H 4 O 4 )? 2. How many moles and grams of malonic acid are needed for 50ml of a 0.15M solution? moles grams (enter this value on the appropriate line under Solution C on page 2)

11 Page 11 of What is the molecular mass of manganese sulfate (MnSO 4 H 2 O)? 4. How many moles and grams of manganese sulfate are needed for 50ml of a 0.02M solution? moles grams (enter this value on the appropriate line under Solution C on page 2)