CHE 321 Midterm: CHE 321 SBU CHE321 Exam 2 16

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turquoisegnu128

31 Jan 2019

School

Stony Brook University

Department

Chemistry

Course

CHE 321

Professor

All

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Document Summary

There are 5 problems on 5 pages in this exam (not counting the cover sheet). You may use a calculator if you wish, provided your calculator does not do calculus. How- ever, it is unlikely to be of much help. Do all of your work in this exam booklet, and cross out any work that the grader should ignore. You may use the backs of pages, but indicate what is where if you expect someone to look at it. Books, extra papers, and discussions with friends are not permitted. If you ask me how to do any of the problems during the test, i"ll be happy to tell you exactly how to do it. But i"m in india, so good luck with that. You have some amount of time to complete this exam. It is like an hour and some more, more or less. For the function f (x, y) = x2 + 2xy + y4, nd all critical points.

Related Questions

The mass of NaHCO3 on the packaged is 1.916g and the experience mass of NaHCO3 is 1.932g. So how do do the post-lab question .#1 explain please

OBJECTIVE:

In this lab, you will produce several different voltaic cells, measure the total voltage, and determine the corresponding half-cells and total potentials. A copper electrode and a copper gluconate electrolyte solution will be used in every system, and four additional half-cells with different metals and salts will be run against it to compare the half-cells and total overall voltage. The salt bridge, consisting of filter paper and potassium chloride, connects the two cells.

PROCEDURE

Put on your goggles and lab apron.

Label five clean, dry 100 mL beakers as copper gluconate, tin sulfate, aluminum sulfate, iron sulfate, and zinc

sulfate.

Using a clean, dry 100 mL graduated cylinder (and pipette if needed), measure and pour 25.0 mL of each

solution into the corresponding beaker. Be sure to thoroughly rinse the graduated cylinder with dH2O before measuring out a new solution. Some of your bottles of solution may need shaken up before using to re- dissolve anything that has settled out. If shaking doesnât completely work, use a clean, dry glass stirring rod to help break up and dissolve. Mix thoroughly. Just be sure to clean the stir rod before moving it to a different solution bottle.

Lightly sand the provided metal strips of copper, tin, aluminum, iron, and zinc down with sand paper to remove the oxidized layer. When finished, wipe off any debris. Set the strips aside. Be sure you have sanded these well to ensure reaction.

Use scissors to cut the filter paper into five strips. Each strip should measure about 20 mm (2 cm) x 150 mm (15 cm).

Note: When your electrochemical cell apparatus is set-up, each strip should be long enough to connect the electrolyte solutions in the two adjacent beakers (half-cells). The strips may look short now, but after they soak, you will see that are the proper length. See pic below.

Using a clean, dry 100 mL graduated cylinder, measure and pour 25.0 mL of potassium chloride solution into a 100 mL beaker. Place the filter paper strips into the beaker. Allow the strips to soak for approximately 5 minutes. This process creates the salt bridge. Be sure all papers are in contact with KCl and get soaked.

Place the metal copper strip in the copper gluconate beaker, allowing it to lean against the side of the beaker.

Repeat Step 7 for each of the metal strips and the corresponding beakers (see pic below).

Place one of the potassium chloride soaked filter papers between the copper gluconate solution and the tin

sulfate solution so that the filter paper is immersed in both solutions simultaneously. The KCl strip must be touching both solutions (itâs OK for the strip to cling to the wall of each beaker, so long as it is touching both solutions)! Pick up the strip by the middle, and drain excess solution by touching the ends to a paper towel. Position the paper strip salt bridge so that it makes contact with both solutions, but is not touching the electrodes.

Assemble the 7-Function Multi-Meter by firmly pressing the positive (red) Test Lead into the V?mA (center) jack. Press the negative (black) Test Lead into the COM (bottom) jack. Rotate the red, center dial until it is pointing towards the DCV 2000 m position (Hint: this position is similar to the northwest position on a compassâsee pic below). 2000 m is equal to 2 volts. That means your read out is now in mV, so you will have to divide the readout by 1,000 when recording your data in V. For example, in the picture below, my readout of 980 mV would be 0.980 V. Turn the multi-meter on.

Touch the red multi-meter probe to the top of the copper strip, and the black multi-meter probe to the top of the tin strip. This works best when you press lightly down on the top edge of the metal strip with the side of the probe, as this creates more constant contact and less fluctuation movement. See pic below. If you happen to have a set of alligator clips to connect the probes to the metal, that would work even better.

Note: The red probe should be placed on copper for all of the half-cell apparatuses. The black probe should be placed on the remaining metals.

Note: Alternatively, you can bend the metal strips to rest over the edge of the beaker, and to give a flat surface that makes it easier to rest the probes:

Record the voltage differential created between the two half-cell in Table 1 (see below). Watch the voltage for a few seconds and then take a reading. Some readings stabilize quickly, others will drift for as long as you want to watch. (Swirling the solution with the electrode can also change the reading.) Typically, waiting a few seconds after connecting the cell yields the most reliable reading.

Take a picture of your voltaic cell. This will be a little tricky since you have to hold the probes, but try to get someone to help. Otherwise, do the best you can. The digital readout on the multimeter must be visible for each sample, and both half cells must be visible and clearly labeled. Your face must be visible in the picture also, as well as a label with your name/student ID, the lab title, and the date. Unless all fourâyour face, name/ID, the lab title, and the date are clear, you will not receive credit. Save this picture, and combine all four voltaic cells (Sn, Al, Fe, Zn) into a single PDF document in order to submit it in the Lab 14 Assignment.

Discard the salt bridge, and repeat Steps 9 -13 three more times using the aluminum sulfate solution, iron sulfate solution, and the zinc sulfate solution. Each solution should be paired with the copper gluconate solution, placing the red probe of the volt meter on the copper each time. A new salt-bridge should be used for each electrochemical cell. Record your data in Table 1 (see below).

Wash your glassware (and plasticware) with warm water and soap. To minimize spotting and mineral contamination, it is best to do a final rinse with dH2O. Dry your glassware (or leave set to dry) and put it away. All chemical solutions used in this lab can now be safely washed down the sink, and solid metals can be thrown away or recycled. Clean up your lab and put away your materials.

RESULTS

Make a data table in either Word or Excel that shows the multi-meter voltage measurements (V), experimental half-cell potential using copper electrode as the standard, the theoretical half-cell voltage, percent error, the half-cell and net cell reactions, and free energy (?G). Your table should look something like the one on page 6 of this document. Also see the description below the sample table for help in determining all of these values. Make sure that the calculation sections shown below are included within your Table 1 document as well.

Save your Table 1 as a PDF file to be submitted with the Lab 14 Assignment. Be sure your document includes your name/student ID, lab title, and date. Unless all threeâyour name/student ID, the lab title, and the date are on the document and legible, you will not receive credit. All columns and values must be clearly labeled.

You should have = 1 PDF of a pic of you and your 4 electrochemical cells being measured (Step 13) + (Steps 16-17). All pictures, tables, and labels must be clear and properly labeled or they will not receive credit.

Question:

You probably observed the cell voltage change slightly while you were observing it. Explain what is occurring at the electrode that causes the observed voltage to change?

I need help writing a purpose for a chemistry experiment. I feel I was always do a great job, but my professor always gives me 3 out of 6. Iâm out of ideas. She wants us to summarize the introduction, and summarize what you will do in each section of the experiment and state how this work will support the goal of this experiment being longer than 1/2 a page. She also said not to go into depth on calculations just summarize each section in procedure. Please, please help me right a better purpose for the experiment below. Thanks.

Experiment 5 Analysis of Copper in a Brass Sample

Introduction

A.General

The purpose of this experiment is to determine the percentage of copper present in an unknown brass sample spectrophotometrically using a Spectronic 20. Brass is an alloy consisting of tin, lead, copper, and zinc. The brass sample has already been ground to a fine powder.

A standard curve (graph) will be made for Cu⁺² in which a plot of absorbance (instrument reading from the Spectronic 20) versus molarity of Cu⁺² for various solutions will be made. These standard Cu⁺² will be made by dissolving the appropriate amount of CuSO₄ * 5H₂O is distilled water. The brass samples will be made by dissolving the brass in concentrated HNO₃ to produce Pb⁺², Cu⁺², Zn⁺² is solution along with the finely divided white hydrated tin (IV) oxide solid. After the properly prepared solutions have been filtered to remove the tin (IV) oxide, the absorbance of these solutions will be measured using the Spectronic 20. The molarity of Cu⁺² in these solutions will be obtained from the standard curve. Then the percent copper in the brass sample will be calculated.

B.Method of Analysis

In each of the 5 solutions (3 for determining the standard curve and 2 for the determination of copper in brass) the molarity of Cu⁺² present will be determined spectrophotometrically using the Bausch & Lomb Spectronic 20. The only species in the solution that absorbs at 620 mÂµ (this wavelength corresponds to the visible portion of the electromagnetic waves) in Cu⁺². In this aqueous solution Cu⁺² is really present as the aquo complex [Cu(H₂O)₆]⁺². The other species Pb⁺², Zn⁺², H⁺, NO₃^-, SO₄^-2, and H₂O do not absorb at the 620 mÂµ wavelength of light (and tin is precipitated as hydrated tin (IV) oxide).

In the Spectronic 20, light of wavelength of 620 mÂµ and certain initial intensity, I₀ is allowed to pass through the sample. The wider the sample tube (width = b) and the greater the molarity of Cu⁺², the more absorption will occur causing the intensity of the 620 mÂµ wavelength (I) to be less as it leaves the solution. The following equation describes the process quantitatively: A = log I₀ / I = abM, where A = absorbance (quantity actually measured by the Spectronic 20), M = molarity of the absorbance species (Cu⁺² in this experiment), b = tube diameter, and a = constant which is characteristic of each absorbing species. Since the same tube (a special tube called a cuvette is used for spectrophotometry) is used throughout the experiment, b remains constant. Hence, we can define a new constant, K = ab. Therefore, the above equation becomes: A = KM = K [Cu⁺²]. A plot of A (the absorbance value measured by the use of the Spectronic 20) versus the molarity of Cu⁺² should be a straight line going through the origin (A = 0.000 and M = 0.000).

Three standard Cu⁺² solutions will be made by dissolving a known amount of CuSO₄ * 5H₂O in distilled water. Hence, the molarity of these solutions will be known. Then the absorbance value for each of these three solutions will be measured using the Spectronic 20. A plot of the three absorbance values versus the corresponding Cu⁺² molarities will be done. The best straight line is drawn through these three points and the origin.

With the standard curve, the molarity of Cu⁺² in any Cu⁺² solution can be determined by measuring the absorbance value of the solution (always using the same cuvette and Spectronic 20) and referring to the standard curve to find the molarity of Cu⁺² that corresponds to that absorbance value. Thus, the molarity of Cu⁺² in the brass sample solutions can be obtained from the standard curve after their absorbance values have been measured on the Spectronic 20.

Procedure

Preparation of Brass Sample Solutions

Weigh two brass samples between 0.6 to 0.8 g (to three d.p.). This is done by putting a small clean, dry beaker (100 or 150 mL) on the pan of the balance. Zero the balance. Carefully add the unknown to the beaker until you get a weight somewhere between 0.6 to 0.8 g (Record to 3 d.p.). To each beaker (under the hood) add 12 mL of 8 M nitric acid very slowly. The reaction of 8 M acid with the finely divided brass sample occurs vigorously. The 8 M must be added slowly to prevent splattering of the brass sample out of the container. The reaction is performed in the hood to prevent toxic brown NO₂ gas from entering the lab. After the initial reaction, leave the beaker in the hood but mix occasionally over 30 mins (during this 30 mins the known CuSO₄ * 5H₂O solutions (Part B) can be measured on the Spectronic 20.)

After the brass has reacted with the 8 M nitric acid, wash down the sides of the beaker with some distilled water from a squeeze bottle. Quantitatively, with no spillage transfer the mixture to a clean 50 mL volumetric flask. After solution is poured into the volumetric flask, add more distilled water to the beaker, washing the sides of the walls. Pour this quantitatively into the volumetric flask as mentioned above. Repeat this two more times. Then add enough distilled water until the 50.00 mL mark is reached by the bottom of the meniscus of the solution. Stopper and mix very well by continually shaking the flask in the upright and inverted positions.

The blue solutions must be filtered to remove the tin (IV) oxide or an incorrect high reading will be obtained for the absorbance reading due to the scatter of the light waves. Filter each of the solutions through dry Whatman No. 1 filter paper. (Use 2 pieces to avoid another filtration. Place one piece on the other. Fold in half and then fold in half again. Open it up into a cone and place it into a filter funnel.) Collect the filtrate in clean, dry beakers. If the filtrate is not a clear pale(blue) color, then another filter again with new Whatman filter paper and with clean, dry beakers. One to three filtrations maybe needed, depending on how much tin was present in the brass sample. Now measure the absorbance of these two solutions as described in the section âUse of the Spectronic 20.â Be sure you do these absorbance measurements and the three from part B on the same Spectronic 20.

Preparation of known CuSO₄ * 5H₂O solutions

The three known (standard solutions have already been prepared for you. They are ready for use in their cuvettes by the side of the Spectronic 20 instrument. These three solutions were prepared for you by the stockroom personnel as described in the procedure in the next paragraph. All you need to do is to measure the absorbance of these three prepared standard solutions.

Weigh three 50 mL volumetric flasks (to 3 d.p.). To each of these volumetric flasks add about 1.000, 2.000, and 3.000 grams of CuSO₄ * 5H₂O. Again weigh the volumetric with the CuSO4 * 5H2O (to 3 d.p.). Add 6.0 mL of concentrated nitric acid (measured with a graduated cylinder) and approx. 25 mL of distilled water to each flask. Mix until CuSO₄ * 5H₂O dissolves. Then add distilled water until the bottom of the meniscus of the solution hits the 50.00ML mark. Stop and mix well (as previous section for making the brass solutions). Measure the absorbance of each solution as described in the section âUse of the Spectronic 20.â Be sure you do these absorbance measurements and the three from part A on the same Spectronic 20.

Use of Spectronic 20

How to use the Spectronic 20 and measure the absorbance of each solution is described on another document I will give. Remember wavelength should be set at 620 mÂµ.

CHE 321 Midterm: CHE 321 SBU CHE321 Exam 2 16

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