Pho.rs: Electrolysis, photosynthesis and spectrometry

A1 ^0.40 Balance the equations for the reactions occurring in the solution at the anode

\[ \mathrm{H_2O} - e^- \to \mathrm{H^+ + O_2} \uparrow \]

and cathode:

\[ \mathrm{Cu^{2+}} + e^- \to \mathrm{Cu^0}.\]

Assume that no other reactions occur at the anode and cathode.

Write down and balance the overall equation for the electrolysis of an aqueous solution of $\rm{CuSO_4}$.

\[ \rm Cu SO_4 + H_2O \to\]

A2 ^1.00 Prepare $V_0=150$ mL of copper sulfate with a molar concentration of $c_0=0.400~\mathrm{M}$. You are given copper sulfate powder ($\rm CuSO_4 \cdot 5 H_2 O$). How many grams $m_{bs}$ of powder are needed to prepare the specified solution? It can be assumed that the volume of the resultant copper sulfate solution is the same as the volume of the added water.

We will call the resulting solution “solution A2.” Pour $5$ mL of solution A2 into Answer tube A2.

A3 ^2.50 In accordance with instruction G2, perform electrolysis of $120$ ml of solution A2 for $t_0=1$ h at a current of $I=1$ A.

Record the dependence of the volume of oxygen released $V_{\rm O_2}$ on time $t$. Take at least 10 measurements. Plot the resulting dependence and draw an approximation curve.

A4 ^0.60 After electrolysis, stir the solution remaining in the electrolyzer. In accordance with the G2 instructions, filter approximately 20-25 mL of the stirred solution after electrolysis.

We will refer to the filtered solution as “solution A4.” Pour 5 ml of solution A4 into Answer tube A4.

A5 ^0.20 Calculate the charge $Q$ that flowed during electrolysis based on the known value of the current.

A6 ^0.30 The amount of oxygen released at point A3 can be used to determine the charge flow during electrolysis. Write a formula that relates the total volume of oxygen released $V_{\rm O_2}$ to the charge flow $Q_{\rm O_2}$. Calculate the numerical value of the charge $Q_{\rm O_2}$. Assume that the experiment takes place at a pressure of $p_0=10^5$ Pa and a temperature of $T_0=298$ K.

B1 ^1.00 In the answer sheets, fill in the table showing what volume of solution A2 ($V_\textbf{A2}$) and water ($V_{\rm H_2O}$) need to be mixed to obtain $4$ mL of the required solutions.

Note that the initial solution A2 has very strong absorption, so in this point you calculate its dilution by a factor of 10 or more.

B2 ^1.50 Using the calculations made in the previous section, prepare five solutions in optical cuvettes. In accordance with instruction G1, measure the absorption spectrum of each of the five solutions.

Save the measured spectra in the “Results/B2” folder on your desktop under the names “B2.cuvette number.txt” (for example, “B2.3.txt”).

B3 ^0.40 Specify the wavelength $\lambda_0$ of light that is most strongly absorbed by $\rm CuSO_4$ solutions.

B4 ^2.00 For each cuvette, record the absorption spectra and the determine the absorption
coefficients $A$ at the wavelength $\lambda_0$ that you have chosen. Plot a graph of the dependence of absorption $A$ on the molar concentration of copper ions $[\rm Cu^{2+}]$, draw a fitting straight line $A=s\cdot [\rm Cu^{2+}]$ and determine its slope $s$.

B5 ^0.30 Measure the absorption spectrum of a 10-fold diluted solution of A4.

Save the measured spectrum in the “Results/B4” folder on your desktop under the name “B4.txt”.

B6 ^0.80 Determine the concentration of copper ions $[\rm Cu^{2+}]_\textbf{A4}$ in solution A4.

B7 ^1.00 The decrease in copper ion concentration in the solution can also be used to determine the charge passed during electrolysis. Write a formula that relates the initial concentration of copper ions $c_0$, the final concentration of copper ions $[\rm Cu^{2+}]_\textbf{A4}$, and the charge flow $Q_{\rm Cu}$. Calculate the numerical value of the charge $Q_{\rm Cu}$. Assume the volume of the solution doesn't change throughout the electrolysis.

C1 ^1.50 At the end of the exercises, there is an enlarged graph (Fig. 11). Determine the absorption values $A_{peak}$ for each $\rm pH$ value on the graph. Determine the absorbance value $A_{iso}$ at the isobestic point. Calculate the ratios $A_{peak}/A_{iso}$ for each $\rm pH$ value. For convenience, a table is provided in the answer sheet. Plot a graph of $A_{peak}/A_{iso} ({\rm pH})$ and draw an approximating curve.

C2 ^0.30 This step uses a thin glass cuvette with an adapter. Following the G1 instructions for thin cuvettes, obtain the absorption spectrum of undiluted solution A4 without indicator. Save the measured spectrum to the “Results/C2” folder on your desktop under the name “C2.txt”.

С3 ^0.30 This step uses a thin glass cuvette with an adapter. Following the G1 instructions for thin cuvettes, obtain the absorption spectrum of the undiluted A4 solution with the indicator. Save the measured spectrum to the “Results/C3” folder on your desktop under the name “C3.txt”.

C4 ^0.30 The introduction to the problem describes how absorption spectra are combined when there are several substances in solution. Based on measurements at points C2-C3, calculate the absorption $A'_{peak}$ at wavelength $\lambda^{CR}_{peak}$ caused only by the absorption of the indicator. What is the absorption $A'_{iso}$ at a wavelength of $\lambda^{CR}_{iso}=475$ nm caused only by the absorption of the indicator?

C5 ^0.50 Based on the data in point C4, calculate the ratio $A'_{peak}/A'_{iso}$. Using the graph from point C1, determine the value of ${\rm pH_{fin}}$ in solution A4.

C6 ^1.40 Fill in the remaining fields in the table on the answer sheets.

С7 ^3.00 Perform the experiment described above, adding the specified amount $\Delta V$ of acid with concentration $C_{\rm HCl}$ at each step. Measure and save the absorption spectrum at each step according to instruction G1. Save the measured spectra in the folder on your desktop named “Results/C7” under the names “C7.step number.txt” (for example, “C7.2.txt”). You should end up with 8 spectra. Pour the remaining solution after obtaining all spectra into Answer tube C7.

C8 ^0.80 Display all spectra from item C7 in the program's working area. Determine the wavelength $\lambda^{BB}_{peak}$ at which absorption changes most significantly with $\rm pH$ changes. Determine the wavelength of the isosbestic point $\lambda^{BB}_{iso}$.

C9 ^1.50 Plot the graph of the dependence of the absorption ratio at wavelengths $\lambda^{BB}_{peak}$ and $\lambda^{BB}_{iso}$ on $\rm pH$ (i.e., the graph $A_{peak}/A_{iso} ({\rm pH})$ for bromophenol blue).

С10 ^0.30 This step uses a thin glass cuvette with an adapter. Following the G1 instructions for thin cuvettes, obtain the absorption spectrum of the undiluted A2 solution without indicator. Save the measured spectrum to the “Results/C10” folder on your desktop under the name “C10.txt”.

С11 ^0.30 This step uses a thin glass cuvette with an adapter. Following the G1 instructions for thin cuvettes, obtain the absorption spectrum of the undiluted A2 solution with indicator. Save the measured spectrum to the “Results/C11” folder on your desktop under the name “C11.txt”.

С12 ^0.30 Based on the measurements in questions C10-C11, calculate the absorption $A'_{peak}$ at wavelength $\lambda^{BB}_{peak}$ caused only by the absorption of the indicator. What is the absorption $A'_{iso}$ at a wavelength of $\lambda^{BB}_{iso}$ nm caused only by the absorption of the indicator?

С13 ^0.50 Based on the data in point C12, calculate the ratio $A'_{peak}/A'_{iso}$. Using the graph from point C9, determine the value of ${\rm pH_{ini}}$ in solution A2.

С14 ^1.00 By increasing the concentration of hydrogen ions in the solution (i.e., decreasing the $\rm pH$), it is also possible to determine the charge passed during electrolysis. Write a formula that relates the initial ${\rm pH_{ini}}$ of the solution, the final ${\rm pH_{fin}}$ of the solution, and the charge $Q_{\rm pH}$ that has passed through. Calculate the numerical value of the charge $Q_{\rm pH}$.

D1 ^0.70 Based on the laws you know, fill in the table on the answer sheet, checking only one of the options for each statement: true/false.

D2 ^0.30 Select and mark with a check mark on the answer sheet the most reliable value of the leaked charge.

E1 ^0.60 For each LED battery, measure the voltage across the 3 LEDs connected in series
and calculate the voltage across single LED when the power source is turned
on. Fill in the table in the answer sheets.

E2 ^0.30 Find the current $I$ flowing through the LEDs of each color. Fill in the table in the
answer sheets.

E3 ^0.30 Find the power of light $P$ emitted by each of the LED. Fill in the table in the
answer sheets.

E4 ^1.00

For this task, use microorganism $A$.

Prepare the setup for measurements according to the G3 instructions.

Turn on the light source and start timing.

If no oxygen release is observed 30 minutes after the start of the experiment, record zero values for $V_{\rm O_2$ in the table in the answer sheets.
If oxygen release is observed 30 minutes after the start of the experiment, continue the experiment for another 1.5 hours. Record the volume of oxygen $V_{\rm O_2}$ released when illuminated by different colors of light in the table on the answer sheet.

E5 ^1.00 For microorganism $B$, repeat the procedure described in the previous question.
Fill in the table in the answer sheets.

E6 ^1.00 According to instruction G4 get the image of the cells in Goryaev's chamber for both microorganisms $A$ and $B$. Save the acquired images to the “Results/E6” folder on your desktop under the name “E6.A.txt” and “E6.B.txt” respectively.

According to instruction G4, use Goryaev's chamber to count the number of cells in the four small squares $n_A$ and $n_B$ of microorganisms $A$ and $B$.

The edge of the large square of the Goryaev chamber is 0.2 mm, the depth of the chamber is 0.1 mm, and the large square consists of 16 small squares. Count the total number of cells $N_A$ and $N_B$ of microorganisms $A$ and $B$ inside a 20 ml syringe. Write down the calculation formula showing how $n_A$ and $N_A$ are related.

E7 ^1.20 Using the data obtained in points E4, E5, and E6, calculate the photosynthetic efficiency $E$ for both microorganisms and all colors of light. Fill in the table in the answer sheets.

E8 ^1.00 Using the data obtained in questions E3 and E7, fill in the table in the answer sheets.

F1 ^2.00 According to instruction G5, perform thin-layer chromatography of extracts of microorganisms $A$ and $B$.

Immediately after completing the chromatography and drying the plate, analyze the table and carefully mark the spots corresponding to chlorophylls with an “X” and the spots corresponding to carotenoids with an “O” on the plate with a pencil.

Raise the HELP sign so that an assistant can come to you and photograph the plate.

Place the marked plate in Answer tube F1.

F2 ^1.00 In accordance with instruction G1, obtain the absorption spectrum of extracts from microorganisms $A$ and $B$.

Save the measured spectra in the folder on the desktop “Results/F2” under the names “F2.A.txt” and “F2.B.txt” for microorganisms $A$ and $B$, respectively.

Pour 3 ml of the microorganism extract solutions you measured into Answer tube F2.A and Answer tube F2.B.

F3 ^1.40 Based on the chromatograms you have obtained, as well as the absorption spectra, mark whether the statements are true or false on the answer sheet.

F4 ^0.80 What conclusions can be drawn from the results of thin-layer chromatography and absorption spectrum analysis? Mark whether the statements are true or false on the answer sheet.

H1 ^0.80 Select the correct statements about microorganisms.

H2 ^0.80

The figure shows a diagram of a small pond with poor water circulation. Identify the zones of the pond (A-D) where the following microorganisms will live.

cyanobacteria and green algae
anaerobic decomposers of organic matter
green bacteria
purple bacteria

Write the numbers of the organisms in the table on the answer sheet.

H3 ^0.80 Based on the information about microorganisms $A$ and $B$ that you obtained during your research, determine in which pond zone (A-D from task H2) each of the microorganisms is most likely to live.

H4 ^1.00 Cyanobacterial mat consists of many phototrophic and non-phototrophic microorganisms that are arranged in layers one below the other. Indicate whether the following statements are true or false.

Electrolysis, photosynthesis and spectrometry

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