Question

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. I’m 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 mµ (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 mµ wavelength of light (and tin is precipitated as hydrated tin (IV) oxide).

In the Spectronic 20, light of wavelength of 620 mµ 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 mµ 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 “Use of the Spectronic 20.” 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 “Use of the Spectronic 20.” 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 mµ.