Question Description
I’m working on a chemistry report and need support to help me understand better.
I need help with Lab Report Below.
Question #1. In your own words, summarize the goals of this experiment.
DATA
Question #2. Summarize the procedure used to prepare the standard solutions; that is, what solutions (identities and concentrations) and volumes were combined to prepare the solutions?
Calculation #1. Calculate the concentration of HIn in Standard Solution A.
Show your work and include units.
Calculation #2. Calculate the concentration of In– in Standard Solution B.
Show work and label units.
Question #3. Complete Report Table 1 below.
Report Table 1. Chemical Species Present and Their Concentrations for Standard Solutions A and B
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standard solution |
species in solution (HIn or In−) |
concentration (M) |
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A |
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B |
Question #4. Record your visual observations of the test tubes in Report Table 2 below. Note the colors of the solutions, and the different degrees of color intensity.
Report Table 2. Volumes of Reagents and Observed Colors for Sample Solutions 1-5 and an Unknown Solution of Bromocresol Green
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solution |
CH3COOH (mL) |
CH3COONa (mL) |
bromocresol green stock solution (NaIn) (mL) |
observed color of solution |
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Sample 1 |
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Sample 2 |
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Sample 3 |
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Sample 4 |
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Sample 5 |
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Unknown |
Question #5. Use the colors of the solutions to predict which of the Sample Solutions 1-5 will have a pH closest to that of the Unknown solution. Explain your answer.
Question #6. Complete Report Table 3 below.
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Report Table 3. Absorbance and pH data
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Question #7. Which Sample Solution (1 – 5) had a pH closest to the measured pH of the Unknown Solution? |
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Question #8. How do the predicted (by color) and measured pH of the unknown solution compare?
DATA ANALYSIS AND RESULTS
Calculate Molar Absorptivities of HIn and In– at 442 nm and 617 nm
Calculation #3. Use the concentration and absorbance data for Standard Solution A to calculate the molar absorptivity of HIn at 442 nm and at 617 nm (i.e., ε(HIn)442 and ε(HIn)617). Enter your results in Report Table 4 below. Show a sample calculation here.
Calculation #4. Use the concentration and absorbance data for Standard Solution B to calculate the molar absorptivity of In‒ at 423 nm and at 617 nm (i.e., ε(In‒)442 and ε(In‒)617). Enter your results in Report Table 4 below. Show a sample calculation here.
Question #9. Complete Report Table 4 below. Refer to Report Tables 1 and 3, and Calculations #3 and #4 above.
Report Table 4. Concentrations, Absorbances, and Molar Absorptivities for Standard Solutions A and B
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Standard Solution |
species in solution (HIn or In‒) |
concentration (M) |
A442 |
A617 |
442 |
617 |
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A |
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B |
Calculate Concentrations in Sample Solutions 1-5 and in Unknown Solution
Use the absorbance data for mixtures of HIn and In– to set up your equations to solve for the concentrations of HIn and In– in each sample solution and the unknown solution. (Review the EXPERIMENTAL DESIGN section for information about how to solve two equations and two unknowns.) Remember that the total absorbance at each wavelength is the sum of the absorbance of HIn and the absorbance of In–. Enter your equations in Question #10 (Report Table 5) below.
Question #10. Complete Report Table 5 below. Note: substitute your numerical values for A and Ɛ; the path length of 1.00 cm can be omitted from the equations.
Report Table 5. Systems of Equations for Sample Solutions 1-5 and Unknown Solution
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solution |
equations |
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general form |
A442 = ε(HIn)442 x [HIn] + ε(In‒)442 x [In‒] A617 = ε(HIn)617 x [HIn] + ε(In‒)617 x [In‒] |
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Sample 1 |
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Sample 2 |
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Sample 3 |
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Sample 4 |
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Sample 5 |
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Unknown |
You must show all your work (including units) for [HIn] and [In−] in Sample Solution 1 in Calculation #5.
You may use Wolfram-Alpha or other calculator to do the remaining calculations and/or to check your work.
- https://www.wolframalpha.com/
- Set up your 2 equations and enter them using the following example format: 85x + 12y = 0.623, 6x + 106y = 0.485 (i.e., 2 equations with 2 variables separated by a comma).
- Click the “=” button.
- You may search “solve a system of linear equations” for more information.
Enter your results for [HIn] and [In−] in Sample Solutions 1-5 and the Unknown in Question #11/Report Table 6.
Calculation #5. Sample calculations of [HIn] and [In–] for Sample Solution #1.(Show all work and include units.)
Calculate Hydrogen Ion Concentration
Calculate the concentration of hydrogen ion, [H+] in those tubes where you measured the pH. Enter your calculation results in Question #11 (Report Table 6 below). Show work for a sample calculation in Calculation #6.
Note that the numerical result of a logarithmic function should have the same number of significant figures as the mantissa (i.e., the number to the right of the decimal point in a logarithm). For example, a pH of 1.58 has two significant figures (two digits to the right of the decimal point), and the H+ ion concentration would be expressed as 2.6 x 10‒2 M (two significant figures).
Calculation #6. Sample calculation of [H+] for Sample Solution #1.(Show all work and include units.)
Calculate Values of K
Calculate the values of the equilibrium constant, K, for Sample Solutions 1–5 and the Unknown Solution. Show a sample calculation of K for Sample Solution 1 in Calculation #7. Enter results for the remaining solutions in Question 11 (Report Table 6).
Calculation #7. Sample calculation of the value of the equilibrium constant, K, in Sample Solution #1.(Show all work.)
Question #11. Enter your results from your calculations of [HIn], [In–], and [H+] in Report Table 6 below.
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Report Table 6. Concentrations and Values for the Equilibrium Constant for Sample Solutions 1-5 and Unknown Solution
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Calculation #8. Calculate your average equilibrium constant (Kave) and the standard deviation of your K values (by using the function =stdev.p(range of K values) in Excel). “The standard deviation is a measure of how widely values are dispersed from the average value (the mean).” Values of K can be considered constant if each K value differs from the average by less than 2 times the standard deviation.
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Average K = __________ at __________ °C Std. Dev. = _______________ |
DISCUSSION AND ANALYSIS
Question #12. Explain whether your experimental data support the hypothesis that the value of the equilibrium constant is a constant at a given temperature. If not, identify possible sources of error in the procedure that might account for the variation. Justify your claims using evidence (i.e., your data).
Question #13. Explain why it was necessary to measure the absorbance of each solution at two different wavelengths.
Question #14. Suppose the measurements for Sample Solutions 1-5 had each been done at a different temperature. Do you have enough information to be able to predict how the value (magnitude) of the equilibrium constant would have been affected, if at all, in each case? If not, describe what additional information you would need to make such predictions.
