CHM110H5 Lecture Notes - Lecture 1: Sodium Hydroxide, Due Date, Sarvepalli Radhakrishnan

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26 Feb 2015

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The purpose of this lab is to figure out the identity of 9 unknown solutions given using only the other solutions as reagents. Also, to compare it to your pre lab chart of the observations you would expect to happen during the reaction between 2 solutions. Observations and data table can be found on the data sheet attached. Identity of the 9 unknown solutions: nh4cl, agno3, nabr, naoh, na2s, kio3, kclo3, na2so4, bacl2. The results of the observations that occurred compared to the prelab questions (in which a table was made to predict the reactions of the solutions) was closely the same with each other. Some reactions were wrong but that was due to the prelab data being a prediction. It is safe to say that the experiment supported our data. A conclusion was able to be made about each unknown solution.

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Need help writing all the nie. Info givin with the page

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.

Introduction: The first section of your lab report will be the introduction. In this paragraph or paragraphs, you are to provide information to the reader so that he or she can understand the purpose of the experiment.

Purpose: This should be a concise statement about the labâs objective.Background: Include a brief summary of the topic you are investigating. Include any information that would be necessary to understand the stated purpose.Findings: State the major result for your experiment.

Procedure: This is the second section in your lab report. This section includes information that the reader would need to repeat your experimental procedure. Do not include any observations or results in this section.

Some questions that you should ask yourself to complete this section include the following. What chemicals did you use? What equipment? How much of each chemical did you use? How long did you perform a step within the procedure?

Observations and Results: This is the third section of your report. In this section, you will communicate what you observed during the experiment. The results section is typically dominated by calculations, tables, and figures. In your tables, label the axes of any graphs

Discussion: This is the fourth section of your report. In this section, you will explain, analyze, and interpret your observations and results. Additionally, you will draw conclusions based on your existing knowledge. This is also where you demonstrate your understanding of the experiment by construing the significance or meaning of the results.

Conclusion: This is the last section of your report and is separate from the discussion section. Within a few sentences, provide a concluding statement about the results of your laboratory. In a scientific publication, this section summarizes the significant aspects and results and identifies implications for future study.

Part 1: Create Standard Curve

For your standard curve, you need to make up 100 ml of each of the following concentrations of salt.

100 parts per million (ppm)500 ppm1,000 ppm5,000 ppm10,000 ppm

Note: 1 ppm = 1 mg / 1 L.

Step 1: Obtain five 250 mL beakers and label them 1â5.

Use the Equipment selection from the toolbar.Select 250 mL beaker from the dropdown menu.Right-click on beaker and select Label.Label the first beaker 1, and write 100 ppm in the Annotated Note area of the dialog box.Repeat with four more beakers, labeling them with numbers 2â5 and the appropriate concentration in ppm.

Step 2: Add 100 mL of water to each beaker.

Right-click on the beaker and select Water.Select Volume Level of 100 mL.Select Room Temperature water.Click OK.

Step 3: Add the appropriate amount of NaCl to each beaker.

Right-click on beaker; select Chemicals.Select Sodium Chloride (NaCl), the last one in the list.Change units to mg. Be sure to do this or your experiment will not turn out correctly.Add the appropriate amount of NaCl to each beaker (see the table below).To mix, right-click on the beaker and select Stirring Rod.

Beaker Number

NaCl Concentration (ppm)

Amount of NaCl to add (mg)

100

500

1,000

100

5,000

500

10,000

1,000

Step 4: Measure the conductivity of standard NaCl solutions.

Select the conductivity meter from the Equipment menu. Right-click on the conductivity meter and select Conductivity Meter Mode and then ATC mode.Place the conductivity meter in the first beaker.Select both the conductivity meter and the beaker by drawing your cursor over both objects. You have selected both items when you have a single dotted-line box around both. Go to Arrange on the toolbar and select Combine. (Alternatively, right-click and select Combine.) This action actually places the conductivity meter in the beaker.Record the value on the conductivity meter.Measure the conductivity of Beakers 2â5 and record your results.

Part 2: Determine Conductivity of Unknown NaCl Solutions

Step 1: Obtain three 250 mL beakers and label them 1Uâ3U.

Use the Equipment selection from the Toolbar.Select 250 mL beaker from the dropdown menu.Right-click on the beaker and select Label.Label the first beaker 1U.Repeat with two more beakers, labeling them 2U and 3U.

Step 2: Add 100 mL of Unknown Brine Solution 1 to Beaker 1U.

Right-click on Beaker 1U.Select the Chemicals menu.Select 100 ml of Unknown Brine Solution 1U.Click Okay.

Step 3: Repeat Step 2 with Unknown Brine Solutions 2 and 3.

Add 100 ml of Unknown Brine Solution 2 to a beaker labeled 2U.Add 100 ml of Unknown Brine Solution 3 to a beaker labeled 3U.

Step 4: Measure the conductivity of Unknown Brine Solution 1.

Select the conductivity meter from the Equipment menu. Right-click on the conductivity meter, and select Conductivity Meter Mode and then ATC mode.Place the conductivity meter in Beaker 1U.Select both the conductivity meter and the beaker by drawing your cursor over both objects. You have selected both items when you have a single dotted-line box around both. Go to Arrange on the toolbar and select Combine. (Alternatively, right-click and select Combine.) This action actually places the conductivity meter in the beaker.Record the value on the conductivity meter.

Step 5: Measure the conductivity of Unknown Brine Solutions 2 and 3.

Repeat Step 4 with Beakers 2U and 3U.Record the values on the conductivity meter.

Part 3: Make Standard Curve

Now, you need to make a graph with the data from your five known samples of NaCl (Tubes 1â5). You will use this graph (called a standard curve) to determine the concentration of the unknown samples of NaCl (Tubes 1Uâ3U).

Step 1: Using Excel or another graphing program, make a graph of NaCl concentration (ppm) versus conductivity (ÂµS).

When making graphs, you want to have the independent variable on the x-axis and the dependent variable on the y-axis.In this experiment, you know the concentration of NaCl in the first five samples, so concentration is the independent variable and is on the x-axis. You donât know the conductivityâit depends on the concentration. So, conductivity is your dependent variable and is on the y-axis.If you are using Excel, be sure to select a scatter graph. Your graph should be a straight line; if your graph is not a straight line, you may have plotted it incorrectly.Be sure to give your graph a title and label the axes. A good title might be "The Dependence of Conductivity on NaCl Concentration."A sample graph is shown below.

Step 2: Follow the steps below created your graph

Arrange your standard curve data (Tubes 1â5) in two columns. The first one should be NaCl Concentration (ppm), and the second one is Conductivity (ÂµS) as shown below.

Salt Concentration (ppm)

Conductivity (ÂµS)

2.022

2.107

2.213

3.029

100

4.002

Highlight and select all data, including the names of each column.Go to the Insert menu in Excel. Under Charts, select Scatter. Select Scatter with Smooth Lines and Markers. Your graph will pop up.Under Chart Tools, go to Layout.Select Axis Titles, Primary Horizontal Axis Title, Title Below Axis and type in Salt Concentration (ppm).Repeat for the Primary Vertical Axis Title, Rotated Title and type in Conductivity (ÂµS). If you canât find the Âµ symbol, just use uS for your units.Under Chart Tools, go to Gridlines.Under Primary Horizontal Axis, select Major and Minor Gridlines.Repeat for the Primary Vertical Axis..Your graph should be plotted on a line graph.

Part 4: Determine Concentrations of Unknown NaCl Solutions

Now you will use your graph to determine the concentration of the three unknown solutions (1Uâ3U).

Find the conductivity value on the y-axis and go over to the line at that value. Then find the corresponding value on the x-axis. For example, if Sample 1U had a conductivity of 3.25 uS, the concentration of NaCl would be about 60 ppm. Nark it on a line graph

Record the approximate NaCl concentration for each unknown (1Uâ3U).

STEP 3: Write Your Lab Report

Follow the directions above to write your lab report. Write your lab report in Microsoft Word.

In your lab report, include the following information.

In the results section, include all your data and a copy of your graph.

Answer the following questions in the discussion section.

What were the concentrations of the three unknown brine solutions (Samples 1U, 2U, and 3U)?Why do you have to adjust the conductivity meter to a standard temperature? (25 Â°C)Why did the conductivity readings increase as the concentration of NaCl increased?Youâve probably heard of hard water. What makes water hard? How could you use a conductivity meter to determine the hardness of water samples?Calculate the molarity of each of the standard NaCl solutions. Be sure to show your work!Remember the following.The molecular weight of NaCl is 58.44 g / mole.1 ppm = 1 mg / 1 L, so 100 ppm = 100 mg / L, and so forth.Molarity = moles / L.

CHM110H5 Lecture Notes - Lecture 1: Sodium Hydroxide, Due Date, Sarvepalli Radhakrishnan

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