PDF Lab 7 QUALITATIVE ANALYSIS - Theochem

Lab 7

QUALITATIVE ANALYSIS

Purpose: In this lab you are going to do some scientific detective work. Your job is to identify what is in an unknown sample. The unknown contains two household chemicals from the "suspect" list. You are going to use deductive reasoning and critical thinking to figure out what is present.

Background: Many students feel they are "forced" to take General Chemistry but don't see the relationship between this course and long-term goals like being a doctor or a mechanical engineer. The goal is not for you to learn to solve a particular type of problem but to develop problem solving skills. After this class, you may never need to state Dalton's Law of Multiple Proportions. However, you will need to apply critical analysis. In this lab, you are going to practice reasoning your way through a chemical puzzle. You will carry out a detailed series of solubility, colorimetric, and effervescence analyses (you will mix stuff in test tubes and see if it dissolves, changes color, or makes bubbles). You will then organize your results into a decision making strategic algorithm (make a flow chart). Then you'll use your strategy to determine the identity of both components of your unknown.

The Usual Suspects The components of your unknown will be chosen from the following list of common household chemicals. These chemicals have been stored in the chemistry stockroom and may be contaminated. IT IS NEVER OK TO TASTE CHEMICALS IN LAB. So do not figure out what your unknown is by tasting it.

1. Sodium Chloride (NaCl) ? table salt (makes stuff salty) 2. Sodium Bicarbonate (NaHCO3) ? baking soda (makes fridges less smelly) 3. Sodium Carbonate (Na2CO3) ? baking powder (used in cookies) 4. Calcium Sulfate (CaSO4) ? dry wall board (used in walls) 5. Sucrose (C12H22O11) ? cane sugar (makes stuff sweet) 6. Fructose (C6H12O6) ?fruit sugar (simple syrup, used in soda) 7. Calcium Carbonate (CaCO3) ? antacid tablets 8. Amylose ((C6H10O5)n) ? starch (used in cooking)

Tips for being a good analytical chemist: Don't mix up your samples. Label your test tubes and keep them straight. Clean your equipment (test tubes and scoopula) thoroughly after each use. Be wary of false positives ? if a result is ambiguous, repeat with a different sample bottle.

Keep masking effects in mind. A masking effect is when one result hides another. If compound A turns a solution brown and compound B turns it light yellow and the solution looks brown then A is present, but B could also be present because you can't see if yellow is present or not.

Procedure ? This is the part you write in your notebook. Work with a partner for this lab.

Part A. You will use a series of tests to identify each of the known compounds. Start with all of the compounds at the top of the flowchart. After each test, divide the remaining compounds into categories. Keep going until you have each compound uniquely identified. The most common mistake is contamination. Be sure to clean your scoopula after every use and use the droppers associated with particular reagents (not the one from the drawers).

1. Solubility test ? Place a small amount (just the tip of the scoopula) of each compound in a separate test tube. Add ~5 mL of solvent (in this case deionized water) and see if any solid dissolves. Then place the correct size stopper on the test tube and shake vigorously for 3 minutes. Let the test tube sit for 1 minute. If the solid settles out of solution, the compound is insoluble. After analyzing all 8 compounds, check results with your TA.

Tests for insoluble compounds: 2. Iodine test ?For each compound that was insoluble, add two drops of iodine solution to the test tube with the water still in it, and record color changes. A reaction with iodine should produce a dark blue/black color. If there is no reaction, the yellow-brown color of I2 will remain. 3. Bubble test ? For any insoluble compounds that do not react with I2, in a clean test tube place a small, dry sample. Carefully dispense vinegar into the test tube and watch closely. Are there any bubbles? At this point, the insoluble compounds should be done (each shows unique behavior). If not, check with your instructor.

Tests for soluble compounds: 4. Bubble test ? Now for each of the soluble compounds, see if they react with the vinegar to form bubbles (use clean test tubes, and small dry samples). 5. Acid/base indicator ? For each of the soluble compounds that form bubbles, clean your test tube and obtain a fresh sample of the compound, dissolve it in a few mL of deionized water, and add 2 drops of phenolphthalein. This indicator turns clear/light pink with acid and dark pink with bases. 6. Benedict's test ? For each of the soluble compounds that do not form bubbles, test with Benedict's solution. This test can be used to diagnose diabetes, because one symptom of diabetes is the presence of glucose in urine. This solution was developed to react with sugars like glucose, but not other sugars. Obtain a fresh sample of compound in a clean test tube and add 1 mL (~ 20 drops) of Benedict's solution (no extra water) and shake. Remove the stopper and place in a warm (not boiling) water bath for 3 minutes. Formation of a red or brown solid indicates a reaction, a clear blue or green solution indicates no reaction. 7. Silver nitrate ? For the compounds that didn't react with Benedict's, obtain a fresh sample of your compound, dissolve it in a few mL of deionized water, and add 4 drops of silver nitrate to the test tube. A positive reaction should form lots of snowflake-like

crystals. A negative reaction will be clear or slightly cloudy. Be sure to write down observations. Each compound should be uniquely identified now.

Part B. Have your instructor or student lab assistant check your results from part A. Then identify your binary unknown. Remember to identify BOTH components of the mixture and be sure to write down your unknown number. Use logic to decide which tests to carry out, and be sure to record your decisions and observations in your notebook. If in doubt, test some of the authentic sample side-by-side to compare. Do NOT just repeat every test you did above.

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