Chemistry 130 Experiment: Salt in Sea Water

Transcription

1 Name Partner Lab Section M Tu W Th F Chemistry 130 Experiment: Salt in Sea Water PART I: HOW MUCH CHLORIDE IS IN THE OCEAN? Sodium chloride, salt, NaCl, is everywhere. It is also an important electrolyte in physiological systems. One common way of measuring the quantity of chloride, the anion component of salt, is to carry out a chemical reaction that makes the chloride precipitate from the aqueous solution. We will be learning more about this phenomenon in class later on, but some compounds are water soluble, and some compounds are not. Obviously, sodium chloride is pretty soluble in water. Silver chloride on the other hand, is not all that soluble in water. By switching the cations from sodium to silver, you can turn the chloride into a solid, filter it and weigh it [as it s silver compound]. The chemical reaction that you are doing is: NaCl (aq) + AgNO3 (aq) <=> AgCl (s) + NaNO3 (aq) The (aq) stands for aqueous and means that the material is soluble. Sodium chloride and silver nitrate are soluble in water. The (s) stands for solid, and means that the material is a solid [not soluble]. Sodium nitrate is soluble but silver chloride is not. An important feature of chemical reactivity has to do with systems like this, where most starting materials and products are soluble but one reagent or product is not. This kind of situation forces the reaction to make the material that is not in solution. [For the most part, something has to be in solution in order to react with other dissolved materials]. By converting all of the chloride ion into the insoluble solid silver salt, you can then filter the material, dry it and weigh it. Using a little math, you can figure out how much chloride was in the water. This is typical analytical process in chemistry, not unlike the methods that people would do in a blood lab or a forensics lab. PROCEDURE: 1. Measure out 15 ml of sea water into a 25 ml graduated cylinder. Your lab instructors will discuss the concept of meniscus with you. 2. Transfer the sea water to a 20 ml test tube. 3. Add 15 drops of silver nitrate solution to the test tube. A white solid should form. This is silver chloride. 4. Place cork firmly on the test tube and shake vigorously for 30 seconds. 5. Place the test tube in a rack while you set up your filtration. 6. Flute a round piece of filter paper. Your instructors will show you how to do this. 7. Place fluted filter paper in a funnel, and place the funnel into an Erlenmeyer flask.

2 8. Pour the contents of you test tube into the filter. Rinse the remaining solid with a minimum amount of additional water. 9. Let this filtration stand while you perform the rest of this lab. You will weigh the dried material later. 10. At your lab instructor s discretion, they might wash your solid with acetone to facilitate drying. You will notice that the solid starts to turn dark as time goes by. This is a unique property of silver chloride. [This is a reaction with room light, and the basis of how photographic film works]. NOTE: At this point your lab instructors will divide the lab into two groups. One half of the lab will do POM POM CHEMISTRY and the other half will do CALCULATOR CONVERSIONS AND MORE. At the appropriate time you will switch and do the other section. By the time you are finished with these two sections, your silver chloride should be dry enough to weigh. 11. Obtain the mass of a piece of clean weighing paper. Record the mass of the paper here: 12. Collect your solid onto the preweighed weighing paper by gently scraping the solid with a spatula. Take care not to scrape so hard that you remove fibers of the filter paper. 13. Obtain the mass of the weighing paper and the silver chloride. Record this number here: 14. Subtract the mass of the weighing paper from the mass of the combination of weighing paper and silver chloride. Record the mass of your silver chloride here: 15. To back calculate how much chloride is in one liter of the salt water do the following calculation. We have not yet covered some of this stuff in the lecture, so do not worry to much about this calculation (BUT... note the dimensional analysis). (X g AgCl). (1 mole AgCl). (1 mole Cl 1- ). (35.45 g Cl 1- ). 1. (1000 ml) = Y g Cl 1- (143.4 g AgCl) (1 mol AgCl) (1 mol Cl 1- ) (15 ml) (1 L) L 16. Record the amount of chloride ion in one liter of solution here:

3 PART II: POM-POM CHEMISTRY Give me an N! N! Give me an a! a! Give me a C! C! Give me an l! l! What do you have? Sodium Chloride!...The crowd cheers... This exercise is designed to give you an intuitive feel of the stoichiometry of ionic compounds. You have a kit that represents various ions.. Let s start simple, real simple... Make the following compounds: NaCl K2S CaCl2 BaO Al2S3 Ca3N2 AlP K3N AlBr3 BaF2 Set up these two compounds: NaCl and KBr Disassemble them and recombine them to make NaBr and KCl... This is pretty easy, isn t it? Now let s try something a little harder... Make a NaBr and a K2S. Try to disassemble them and make Na2S and KBr. You run into a problem don t you? But if you start with 2 NaBr s and 1 K2S, you can make it work. You can make 1 Na2S and 2 KBr s. So you need more than one of some of the things. This is what we do when we balance reactions. 2 NaBr + K2S <=> Na2S + 2 KBr Try to set up the following reactions. First make the materials on the left hand side of the equation, and then disassemble them and make the materials on the right hand side of the equation. Imagine that in some strange way, this is what happens at the atomic level.

4 a. Ca3P2 + 3 Na2S <=> 3 CaS + 2 Na3P b. BaS + 2 NaF <=> BaF2 + Na2S c. Al2O3 + 3 K2S <=> Al2S3 + 3 K2O Now for the hardest part... Here are some unbalanced reactions. Figure out how many of each component you need to make them work out. If you have any doubts or problems with this, make sure that you check with your instructor. a. NaF + Al2O3 <=> Na2O + AlF 3 b. AlN + K2O <=> Al2O3 + K3N c. Na 3 N + CaS <=> Ca3N2 + Na2S d. CaF2 + AlCl3 <=> CaCl2 + AlF 3 PART III: CALCULATOR CONVERSIONS AND MORE Perform the following conversions on your calculator: a. Convert 33 g into lbs. [1 lb = 454 g] b. Convert 2.36 x 10 4 cm into km.

5 c. Convert 3.5 x 10-6 ml into gallons. d. Convert 7.2 miles per hour into meters per second. e. Convert 1.3 x 10-7 pounds per gallon into grams per liter. Write isotopic symbols for the following a. An atom with 8 protons and 10 neutrons b. Barium with 5 neutrons [not real, but do it anyway] c. Sulfur-31 d. Carbon-13 How many protons and neutrons are in the following? a K b F c Al

6 Give the symbol for the following ions a. 13 protons and 10 electrons b. 20 protons and 18 electrons c. 9 protons and 10 electrons d. 1 proton and 0 electrons e. 12 protons and 10 electrons How many protons and electrons do the following ions have? a. Ca 2+ b. N 3- c. Ba 2+ d. C 4+ e. O 2-