LABORATORY ACTIVITY 2: STUDENT VERSION Activity 2: Determination of the Salinity of Seawater Introduction This method involves chemical volumetric analysis. A silver nitrate (AgNO 3 ) solution of known concentration is used to precipitate out the chlorides in a seawater sample. The procedure is complicated by the fact that the dense white precipitate, silver chloride (AgCl), does not settle rapidly. It is impossible to tell when all the chlorides have been removed from the seawater unless an indicator is used to signal complete precipitation by a visual color change. The indicator in the analysis is chromate ion. When all of the chloride ion is exhausted, the chromate ion reacts with silver ions and produces silver chromate, which is red. The instant a permanent orange tinge appears in the solution (one that doesn’t vanish with mixing), the addition of silver nitrate is stopped. The final solution color should look like that of orange juice. The chemical reactions are:
The titration is carried out in a 150-mL Erlenmeyer flask, using a white background and good lighting, but away from direct sunlight. Constant but gentle stirring is essential, since the silver chloride tends to form clots, which trap some of the reagents. A magnetic stirrer is very helpful. The silver nitrate is added until the faint reddish or orange tinge remains after stirring for 45 s. At this point one drop should give the desired end point. Observable flocs of silver chloride will obscure the end point, since the solutions that they contain in their interstices will leak out slowly and cause the end point to be impermanent. A stirring rod left permanently in the flask, especially if a magnetic stirrer is not used, will help to break up these curds. Purpose 1. To determine the chlorinity (and thus the salinity, since they are directly related) of a sample of seawater. 2. To use the titration technique with a reaction different from an acid/base reaction. Safety Wear protective goggles throughout the laboratory activity. Avoid getting AgNO 3 on hands since it stains the skin. Procedure 1. With a pipet deliver 5.0mL seawater into a 150-mL Erlenmeyer flask. 2. Using a 10-mL graduated cylinder, add 5.0mL potassium chromate indicator solution, and begin to add the silver nitrate solution from a buret while constantly stirring. 3. When the solution turns orange, stop adding silver nitrate. The color should persist for less than 20 sec. Continue adding silver nitrate dropwise, mixing all the while, until orange color persists. Record the volume of silver nitrate added. 4. Thoroughly wash your hands before leaving the laboratory. Data Analysis and Concept Development From a simple algebraic proportion one can determine the chlorinity of the seawater because it has been shown experimentally that the salinity is a constant 1.80655 times higher than the chloride level. Thus, the salinity (S) can be calculated (using the equation S ppt = 1.80655 Cl ppt; the chlorinity (Cl) of the artificial seawater should be about 19.05 ppt). 1. What is the number of moles of silver used in the titration (AgNO 3 is 0.2184 M)? 2. Determine the number of moles of chloride in the sample. 3. Determine the grams of chloride present in the sample. 4. What is the chlorinity of 10g of sample? (The density of seawater is about 1.05 g / mL 1 ). 5. What is the salinity of the sample in ppt? LABORATORY ACTIVITY 2: TEACHER NOTES Activity 2: Determination of the Salinity of Seawater Major Chemical Concept Quantitative determination of the chloride concentration in a sample of seawater. Level General high school chemistry Expected Student Background Students should have done titrations and titration calculations. Time 55 min Safety Read the Safety Considerations in the Student Version. Materials (For 24 students working in pairs) Nonconsumables 12 Burets 12 Buret stands and clamps 12 Graduated cylinders, 10-mL 12 Erlenmeyer flasks, 150-mL 12 Magnetic stirrers (optional) 12 Pipets, transfer, 10-mL 2 Storage bottles, 1-L 1 Volumetric flask, 1-L Consumables Distilled water (in wash bottle) Indicator solution (3.5 g, potassium chromate, K 2 CrO 4 , reagent grade, per liter solution) 0.2184 M Silver nitrate, AgNO 3 (37.11 g AgNO 3 , reagent grade, to 1.000 L in a 1-L volumetric flask. Store solution in a tightly capped brown glass bottle.) Advance Preparation Prepare: Potassium chromate (observe safety precautions) Silver nitrate (observe safety precautions) Pre-Laboratory Discussion 1. Demonstrate the proper titration techniques and remind students that silver nitrate will stain their hands. 2. Discuss the reaction that gives the red-brown color
and demonstrate a titration showing the students the final end point color. 3. Review the proper use of a transfer pipet. Teacher-Student Interaction Circulate in the laboratory. Ask questions comparing this titration with previously done acid-base titrations. Ask why it is not important to use an exact amount of indicator solution. Post-Laboratory Discussion Part A Discuss differential solubility. Since the Ag 2 CrO 4 (s) is much more soluble than AgCl(s), it is only after almost all of the AgCl(s) is precipitated that the red-brown end point be reached with the precipitation of Ag 2 CrO 4 (s). Anticipated Student Results
Assessing Laboratory Learning 1. If 24.0 mL of 0.250 M AgNO 3 is required to reach the end point of the seawater solution (CrO 4 2 (aq) reaction with Ag + (aq) to give a red-brown precipitate), determine the number of moles of Cl ions present.
2. Determine the grams of chloride present in the sample.
3. What is the chlorinity of a 10.0 mL sample? (The density of the seawater is determined to be 1.05 g/mL.)
4. What is the salinity of the sample? (1.80655) Chlorinity (ppt) = Salinity (ppt)
CAUTION: Use appropriate safety guidelines in performing demonstrations.
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