It is easier to prepare a stock solution of 1dm-3mol (1M) copper sulfate solution and then use the dilution method to prepare the different concentrations (see sample calculations)
Preparation of 1.0M Zinc Sulfate hepta-hydrate solution:
The same calculation can be done to find the mass of zinc sulfate which is needed to prepare 1dm-3mol (1M) ZnSO4 solution.
1. Find the number of moles needed to make 50 mL (0.050 L) of 1.0 M solution using a 50mL volumetric flask. Record your answer. Molar mass of zinc sulfate hepta-hydrate is 287.53g/mol
2. From the molar mass of zinc sulfate hepta-hydrate, ZnSO4.7H2O and the number of moles from Step 1, find the mass in grams of solute needed. Record your answer.
Mass= molar mass number of moles
Mass = 287.53 1 = 287.53 g
This mass is required to prepare 1L of zinc sulfate solution.
For 50ml of 1.0M zinc sulfate solution; divide the mass by 20
The required mass = = 14.377g
3. Weigh 14.377g of zinc sulfate hepta-hydrate using 0.01 (±0.005) digital balance and carefully pour the zinc sulfate to the 50 mL volumetric flask.
4. Add about half the volume (25ml) of distilled water needed and swirl the flask. When most of the solid has dissolved add the rest of the water stopping below the mark on the flask. To add the remaining water use the water wash bottle.
Preparation of concentrated potassium chloride salt bridge (KCl)
- Obtain a filter paper (Salt Bridge) and fully immerse it in the potassium chloride solution of (1.0 M) concentration.
** This is an alternate method that can be used if the U- shaped glass tubing is not available to connect the two half-cells.
Calculation:
1M of KCl solution can be prepared in the same way of the previous solutions. Molar mass of KCl is 74.55 g/mol. This mass is for preparing 1L which is the volume needed for 5 voltaic cells (5 concentrations of copper sulfate).
2- After sitting all of the apparatus, immerse the filter paper in 1M potassium chloride solution.
3- When all of the parts of the voltaic cell are set, place the filter paper between the two beakers (A & B), make sure that the two endings are touching or beneath the surface of each solution.
Part B
1. Obtain two 100 mL beakers labeling the first beaker “A” and the second beaker “B”.
- Using a graduated cylinder, measure 50 mL of Copper (II) sulfate solution (0.2M) and pour it off carefully into beaker “A”
- Measure 50 mL of Zinc (II) sulfate solution (1 M) using a graduated cylinder and pour it off carefully into beaker “B”
- Immerse the filter paper in the potassium chloride solution as a salt bridge.
- Set up equipment to be used for the first trial
Part B.1: Potential difference between Zn and Cu metals
- Place the Cu electrode into the copper solution (beaker A) and the Zn electrode into the zinc solution (beaker B).
- Use the insulated connecting wires to connect each metal electrode to on ending of the voltmeter, the negative to Zn electrode and the positive to the Cu electrode.
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Set equipment as shown in Diagram 1 & 2
- Place the salt bridge (filter paper) between the two beakers (A & B), and make sure that the two endings are touching the surface of each solution.
- Observe and record the readings of the cell’s voltage in the data table.
- Observe and record any change in colors of the solution or electrodes.
- Detach the voltmeter and rinse the electrodes using distilled water.
- Repeat these steps two more times to get more accurate results
** Repeat these steps using different concentrations of copper sulfate each time (0.4, 0.6, 0.8, 1.0 dm-3mol (M)), make sure to change the filter paper and zinc sulfate solution each time.
Method of Data Collection
In terms of methodology, this data table represents the data collected of the readings present from the voltmeter for the first trial of the experiment. It has two main sections: the copper sulfate solution’s concentrations (M) and the experimental voltage (the experiment’s +results).Another similar data table can be developed for the second and third trials of the experiment.
From the results of data table above, plot a graph that represents the change in potential difference in the voltaic cell vs. the different concentrations of copper sulfate solution using the average of the three trials. This is demonstrated below using a scatter graph.
Safety Requirements:
Copper Sulfate pentahydrate Material Safety Data Sheet (MSDS)
Copper Sulfate pentahydrate is known as a hazardous chemical. Avoid direct skin contact (irritant), eye contact (irritant), ingestion and inhalation.
Do not ingest. Do not breathe dust. Wear suitable protective clothing. In case of insufficient ventilation, wear suitable respiratory equipment. If ingested, seek medical advice immediately and show the container or the label. Avoid contact with skin and eyes. Keep away from incompatibles such as metals, alkalis.
Personal Protection:
Wear safety goggles, lab coat and latex gloves.
Waste Disposal:
Copper sulfate is poisonous and harmful to the environment; it should be disposed in the waste container not in the bin or the sink.
Zinc Sulfate Hepta hydrate (MSDS)
Zinc sulfate hepta- hydrate has the same harmful and poisonous effect. The same precautions of copper sulfate should be applied.
Potassium chloride is safe to be used and can be discarded in the sink.
Data Collection
Qualitative Data
The colour of CuSO4 solution is blue. It takes a long period of time for the copper sulfate to dissolve in distilled water. The least concentration of the copper sulfate solution (0.2 M) has the lightest shade of blue while the most concentration copper sulfate solution (1.0 M) has the darkest shade of blue colour. When adding distilled water to a copper sulfate solution it doesn’t mix completely with the solution instantaneously. When placing the copper sulfate solution in the volumetric flask, it shows a more accurate reading of the volume of the solution than when placed in a glass beaker. Zinc Sulfate dissolves easily in distilled water. The colour of the Zinc Sulfate solution remains transparent when distilled water is added to zinc sulfate. No reaction occurs when the copper electrode is added to the copper sulfate solution and no reaction occurs when adding zinc electrode to zinc sulfate solution. When placing the insulated connective wires to electrodes, the potential difference remains zero on the voltmeter. Filter paper had to be rolled and completely immersed in potassium chloride solution before placing it between the two beakers (two half cells). Voltmeter shows readings only when the salt bridge is placed to connect the two half cells. The voltaic cell of 1.0 M copper sulfate solution records the highest voltage. Throughout the experiment, there was no change in the color of copper sulfate or zinc sulfate solutions. The color of zinc and copper metals (electrodes) remained the same, gray and golden.
Quantitative Data
This data table represents the results recorded when conducting the first trial of the experiment for the potential difference using different concentrations of copper sulfate solution.
As shown from the data table above, the copper sulphate solutions’ concentration is directly proportional to the potential difference between two half-cells. Hence, we can see that as the concentration of copper sulphate increases the potential difference between the two half-cells increases.
Data Processing
The processed data will be represented as a scatter graph for the concentration of the copper sulfate solution (M) versus experimental voltage (potential difference) of the two half-cells (V). Sample calculations will be provided to show the calculations needed for the preparation of copper sulfate penta-hydrate solutions, preparation of zinc sulfate hepta-hydrate solutions, the preparation of the salt bride, uncertainties and the percentage error. The formula C1.V1 = C2.V2 will be used throughout the representation of the calculations.
This scatter graph represents the potential difference in Voltaic Cells using five different concentrations of copper sulfate solutions throughout the experiment.
The scatter and line graphs represent the change in potential difference throughout the experiment using different copper sulfate concentrations. The most diluted concentration of copper sulfate (0.2 M) shows the lowest potential difference (voltage) between the two half-cells. In contrast, the most concentrated copper sulfate solution (1.0 m) shows the highest potential difference between the two-half cells. However, potential difference remained constant (0.83 V) for two concentrations of copper sulfate solutions (0.6 M, 0.8 M). Therefore, from the scatter graph above there is a direct relationship between the concentrations of copper sulfate solution and the potential difference. It also proves that the hypothesis stated before is correct as it clearly shows that both are directly proportional. As the concentration of copper sulfate solution increases, the potential difference (voltage) between the two half cells increases.
Sample Calculations
Preparation of Copper sulfate Penta-hydrate solution
To prepare 50 ml of 1dm-3mol (M) copper sulfate solution, use the formula: Mass = number of moles molar mass
The molar mass of copper sulfate pentahydrate is 249.71g/mol.
From the molar mass of copper sulfate pentahydrate, CuSO4.5H2O and the number of moles we can find the mass in grams of solute needed.
Mass of copper sulfate pentahydrate = 1 249.71 = 249.71 g
This mass is required to prepare 1L of 1M copper sulfate solution.
For 50ml of copper sulfate solution, divide the 1L mass by 20 = = 12.485g of CuSO4.5H2O
Dissolve 12.485g of CuSO4.5H2O in 50 ml distilled water, using a volumetric flask.
Use the formula: C1V1 = C2V2 to prepare different concentrations of copper sulfate from the stock solution of 1M concentration.
C1 is the initial concentration needed (M) and V1 is the volume needed for the initial concentration. C2 is the final concentration needed and V2 is the final volume needed for this concentration. Each time complete the volume to 50 ml using distilled water.
Example: preparation of 0.8 M of copper sulfate
Using the formula: C1V1 = C2V2
C1 is: 1 M; which is the initial concentration that we have already prepared, V1 is the unknown, C2 is the 1concentration that we want to have by the end of this dilution which is 0.8 M and V2 is 50 ml which is the total volume that we must achieve each time we prepare a certain concentration for this experiment.
This can be written as:
1 V1 = 0.8 50
= 40
The volume needed of 1.0 M copper sulfate solution is 40 ml.
Hence, complete it by adding 10 ml of distilled water to achieve 50 ml of 0.8 M copper sulfate solution. This can be done for the preparation of the other copper sulfate concentrations. The total volume of 1 M copper sulfate that should be prepared is 150 ml.
This data table represents the preparation of the other copper sulfate concentrations.
Preparation of Zinc Sulfate Hepta-hydrate
The mass of zinc sulfate needed can also be calculated using the formula: Mass = number of moles molar mass. Zinc sulfate salt can be as mono hydrate or hepta hydrate, read the formula on the package for correct calculations.
To prepare 50 ml of 1.0 M zinc sulfate 14.377 g of ZnSO4.7H2O is needed to be dissolved in 50 ml distilled water.
For 5 concentrations of copper sulfate solution, the total volume of 1M of zinc sulfate needed is 250 ml.
So, the amount of zinc sulfate needed is: 14.377 5= 71.883g
Hence, dissolve 71.883g of zinc sulfate in 250 ml distilled water, using 250ml volumetric flask.
Preparation of Potassium Chloride (KCl) solution
To prepare the salt bridge solution; 250 ml of 1M KCl is needed for the 5 sequences.
The amount of KCl needed for 50 ml is 3.72 g.
Hence, dissolve 18.638 g of potassium chloride in 250 ml distilled water using 250ml volumetric flask for total volume.
Uncertainties
Absolute Uncertainties
The absolute uncertainty is the size of the range of values in which the true value of the measurement probably lies.
Graduated cylinder 50ml,
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The total uncertainty for the graduated cylinder is 7.5 ml. This is because throughout the experiment, the graduated cylinder used could only measure up to 50. The graduated cylinder was used 5 times for copper sulfate solutions, used 5 times for zinc sulfate solutions and 5 times for KCl solution during the five steps (measurements) of the experiment. So the uncertainty for the for the preparation of copper sulfate solution is , preparation of zinc sulfate solutions is and for the salt bridge is Hence, the total uncertainty is (2.5 + 2.5 + 2.5).
Volumetric flask uncertainty
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The total uncertainty for the volumetric flask is. This is because, throughout the experiment, a 250 ml volumetric flask was used for zinc sulfate solution, a 250 ml flask was used for potassium chloride and a 50 ml, and 100 ml volumetric flasks were used for copper sulfate solution. So the uncertainty for the volumetric flask of zinc sulfate is, for the potassium chloride solution is and for the copper sulfate solution it is and. Therefore, the total uncertainty is: 0.1 + 0.1 + 0.08 + 0.05 = 0.33 ml. (Wellesley College, 2010)
Digital Scale uncertainty,
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The total uncertainty for the digital scale is. This is because the digital scale’s uncertainty is ±0.005 g. It was used for times; twice for copper sulfate solution preparation, once for zinc sulfate solution and once for the preparation of potassium chloride solution. Therefore, 0.005 4 = 0.02 g.
Voltmeter uncertainty,
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The total uncertainty for the voltmeter is ±0.25 V. The uncertainty for the voltmeter is ±0.05 V, the voltmeter was used 5 times for the 5 different concentrations of copper sulfate. Hence, 0.05 5= 0.25 V.
Percentage Error
An error is the difference between a predicted value and the actual value. Percentage error is the measure of how close you came to the actual answer in a percentage form. Percentage error can be calculated using this formula:
Error =
The theoretical value of potential difference between copper electrode in 1.0 M of copper sulfate and zinc electrode in 1.0 M of zinc sulfate is 1.10 V while the Experimental Value is 0.85 V (UC Davis ChemWiki, 2011). Therefore, the percentage error is:
Error=
Error = × 100 = 22.72
Percentage error = 22.72%
The percentage errors for the other copper sulfate concentrations cannot be calculated as there aren’t any sufficient standard electrode potentials.
Conclusion
The purpose of this experiment was to examine the extent to which the use of different concentrations of copper sulfate solution can have on the potential difference in a Voltaic Cell. Two half-cells were used in the experiment; copper sulfate and zinc sulfate. This is a redox reaction, reduction is: Cu2+ (aq) + 2e- → Cu (s) and the oxidation is: Zn (s) → Zn2+ + 2e- . The sum of the redox reaction for the experiment is: Cu2+ (aq) + Zn (s) → Cu (s) + Zn2+.
Based on the experimental measurements, the most dilute concentration of copper sulfate solution (0.2 M) showed the lowest potential difference between the two half cells (0.80 V). On the other hand, the most concentrated copper sulfate concentration (1.0 M) showed the highest potential difference reading on the voltmeter (0.85 V). Furthermore, the scatter graph and the line graph show the mathematical relationship that exists between the copper sulfate concentration and the change of potential difference between the two half-cells. However, the readings on the voltmeter showed that the copper sulfate concentrations (0.6 M and 0.8 M) had the same potential difference between the half-cells (0.83). This was also represented in the scatter graph as it showed that the potential difference for the 2concentrations remained constant. The percent of error for the difference between the accepted value and the experimental value is 22.72% which is moderately accepted for manipulating the experiment for the first time.
Evaluation
The method used to determine the effect of using different copper sulfate concentrations on the voltages is a fair method. However, there were few major weaknesses for the method used. The voltmeter used for this experiment didn’t represent accurate readings as a trial sometimes would be repeated more than once to be certain of one reading for a certain concentration. A filter paper was used in this experiment as the salt bridge between the two half cells; this can lead to an increase in the percentage error of the experiment as it might show slightly different readings. Since different thickness and diameters of filter papers can record different readings and measurements of the potential difference. The experiment was done throughout the week as it took a long period of time for the preparation of the solutions and salt bridge needed; this can have an effect on the readings for the experiment as the solutions were placed in either volumetric flasks or glass beakers for a long period of time; evaporation may occur which will affect the concentration of the solutions. Due to the large amounts of copper sulfate and zinc sulfate used throughout the experiment, there was no sufficient amount of it to repeat the experiment. Also, it was a way to reduce the amount of waste of copper sulfate and zinc sulfate throughout the experiment. Time shortage decreases the accuracy of readings and therefore, increases the percentage error of the experiment.
Improvements
For more accurate results;
- Use a Differential Voltage Probe (measured in low and high voltages) and the Lab Quest machine for data collection instead of a voltmeter. The Differential Voltage Probe is more accurate than the regular voltmeter.
- Use of a 50 ml volumetric pipette instead of a 50 ml graduated cylinder to use 50ml of copper sulfate, zinc sulfate and potassium chloride solutions during the experiment. This will decrease the uncertainty and the percentage error when using a volumetric pipette.
- Use of U-shaped glass tubing for the salt bridge instead of a filter paper. Using U-shaped glass tubing as a salt bridge can assure that there’s enough KCl needed instead of making sure that the filter paper is fully immersed in the solution. This will allow more passage of ions and electrons through the bridge and can give more accurate reading as there is the same amount of KCl used for each different concentration of copper sulfate. . Furthermore, the way of folding the filter paper won’t be the same each time.
- Decrease the total volume for each solution to 25 ml instead of 50 ml. By decreasing the total volume of each solution to 25 ml, this would require fewer amounts of copper sulfate and zinc sulfate for each concentration and would decrease the amount of time taken for the preparations of the solutions and the amount of waste, too. Hence, decrease in percentage error as the solutions won’t be stored for a long period of time.
- Prepare extra 10 ml of copper sulfate and zinc sulfate solutions to avoid shortage of the solutions while running the experiment. A variation in measuring the volume occurs between the graduated cylinder and the volumetric flask which is the most accurate
- It is important to know the exact amount of solutions needed in order to reduce the waste of copper sulfate and zinc sulfate solutions because they are toxic and harmful to the environment.
References
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Adilia James, S. C. (2010). Wellesley College Intro Chem Lab Manual. Retrieved October 26, 2011, from Wellesley College Website : http://www.wellesley.edu/Chemistry/Chem105manual/Appendices/uncertainty_volumetric.html
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G R Delpierre, B. T. (2011). Electronic Science Tutor . Retrieved October 28, 2011, from Electronic Science Tutor Web Site: http://www.physchem.co.za/OB12-che/daniel.htm
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Neuss, G. (2007). Chemistry for the IB Diploma: Study Guide. New York: Oxford University Press.
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Singh S, G. D. (2011). UC Davis ChemWiki . Retrieved October 22, 2011, from ChemWiki Web Site : http://chemwiki.ucdavis.edu/Analytical_Chemistry/Electrochemistry/Voltaic_Cells
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