152 practice final
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Feb 20, 2024
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1 Name (PRINT) _________________________________________ Student ID ______________________ Chem 152 FINAL EXAM This practice exam is the same as the final exam given in Spring 2021.
Name (PRINT) _________________________________________ Student ID ______________________ 2 1. (27 pts total) Shown below is the cooling curve for 0.7135 g of MgO of unknown purity
reacting with a solution of excess HCl in a calorimeter (the impurities do not react with HCl).
The overall heat capacity of the solution is 311.0 J/°C.
1.1 (8 pts) Assuming no loss of heat to the environment, calculate the total heat absorbed by the solution (Q
solution
, in kJ). Show all work and enter your answer with the correct sign convention and number of significant figures in the box provided. 1.2 (6 pts) Using the heat of reaction given below, calculate the mass of MgO in the unknown. Show all work and enter your answer with the correct number of significant figures in the box provided. MgO (s) + 2 H
+
(aq) →
Mg
2+
(aq) + H
2
O (l) Δ
Hrxn = −
436.8 kJ/mol kJ g
Name (PRINT) _________________________________________ Student ID ______________________ 3 1.3 (4 pts) What is the percent purity of the MgO sample? Show all work and enter your answer with the correct number of significant figures in the box provided. 1.4 (9 pts) A second trial of the experiment was performed using 1.5000 g of the MgO unknown instead of 0.7135 g. Assuming HCl was still the excess reagent and the new mass was factored into calculations, circle the correct words to complete the statements below. a. The change in temperature for the second trial would be ( greater than , less than , equal to ) that of the first trial. b. The amount of heat evolved in the second trial would be ( greater than , less than , equal to ) the heat evolved in the first trial. c. The percent purity calculated from the second trial would be ( greater than , less than , equal to ) the percent purity calculated from the first trial. 2. (32 pts total) To obtain the solubility product, Ksp, of Ba(OH)
2
, a student stirs an excess amount of solid barium hydroxide in water, filters away the un-dissolved solid, and titrates 10.00 mL of the filtrate (a saturated Ba(OH)
2
solution) with 0.1526 M HCl. The student performs 4 trials and the volumes of HCl used in titrations are 20.62 mL, 21.22 mL, 20.88 mL, and 20.55 mL (average volume = 20.82 mL). 2.1 (4 pts) Listed below are four acid-base indicators available in the lab and their pKa values. Circle the indicator that is most appropriate for this titration. a. Methyl orange (pKa = 3.8) b. Bromocresol green (pKa = 4.5) c. Brillant yellow (pKa = 7.2) d. Phenolphthalein (pKa = 9.0) 2.2 (6 pts) Calculate the average deviation of the titration volume. Report your answer as “average volume ± average deviation” using correct number of significant figures. % ( + ) mL
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Name (PRINT) _________________________________________ Student ID ______________________ 4 2.3 (6 pts) The student suspects the titration volume “21.22 mL” is inaccurate. Using a Q-
test with the table of 90% confidence rejection quotients below, determine whether the trial should be retained or discarded. Circle the correct answer below and show all supporting work. # of measurements 3 4 5 6 7 Q0.90 0.94 0.76 0.64 0.56 0.51 Circle the correct answer: The data point ’21.22 mL’ should be ( retained / discarded ). Supporting work: 2.4 (10 pts) Using the average titration volume of HCl given in the problem, calculate the Ksp of Ba(OH)
2
. Show your work and enter your answer in the box.
Name (PRINT) _________________________________________ Student ID ______________________ 5 2.5 (BONUS) Calculate the molar solubility of Ba(OH)
2
in 0.015 M NaOH solution. 3. (34 pts total) 0.1238 g of an Fe(II) unknown sample is dissolved in 7.00 mL of 0.00502 M potassium dichromate solution in a 25-mL volumetric flask. 2 M H
2
SO
4
solution is added to fill the volumetric flask to the calibration mark. The resultant solution appears to be yellow. The balanced reaction equation of the reaction is: Cr
2
O
7
2-
+ 6 Fe
2+
+ 14 H
+
(aq) →
2 Cr
3+
+ 6 Fe
3+
+7 H
2
O A 1.00 mL aliquot of the solution in the volumetric flask is loaded onto a cation exchange column and eluted with 2M H
2
SO
4
. The eluent is collected in 2.00-mL fractions in a 24-well plate. 3.1 (6 pts) In the box below, enter all the cations and anions contained in the 1.00 mL sample loaded onto the column and circle the ion(s) last eluted from the column. You may ignore the counterions from the unknown sample. 3.2 (10 pts) The first 6 wells of the eluent appear yellowish. The absorbances of these fractions are measured at 447 nm without dilution. The results are: 0.010, 0.024, 0.057, 0.057, 0.024 and 0.010. The molar absorptivity of dichromate in 2M sulfuric acid is 350 M
-1
cm
-1
and the cuvettes are oriented properly. Calculate the total moles of dichromate eluted from the column. Show your work and enter your answer in the box. mol M
Name (PRINT) _________________________________________ Student ID ______________________ 6 3.3 (8 pts) Calculate the total moles of dichromate consumed in reaction in the volumetric flask. Show your work and enter your answer in the box. 3.4 (6 pts) Calculate the mass percentage of the Fe(II) in the unknown sample. Show your work and enter your answer in the box. 3.5 (4 pts) Jane performed this experiment. However, she placed the blank cuvette in the wrong orientation and did everything else correctly. How will the result be affected? Circle the correct answer below. Jane’s mass percentage of the Fe(II) in the unknown sample would be (
higher than / lower than / the same as
) the true value. 4. (16 pts total) Solution (A) consists of 7.0 mL of 0.010M K
3
Fe(CN)
6
and 3.0 mL of 0.010M K
4
Fe(CN)
6
and 11.0 mL of de-ionized water. Solution (B) consists of 3.0 mL of 0.010M K
3
Fe(CN)
6
and 8.0 mL of 0.010M K
4
Fe(CN)
6
and 10.0 mL of de-ionized water. Solution (A) is added to the left side of a H-cell, while solution (B) is added to the right side. Graphite rods are used as the electrodes and the LabQuest meter was connected correctly. 4.1 (4 pts) Calculate the initial
concentration of potassium ferrocyanide in the left cell. Show your work and enter your answer in the box. mol % M
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Name (PRINT) _________________________________________ Student ID ______________________ 7 4.2 (8 pts) The standard reduction potential of the half-reaction: [Fe(CN)
6
]
3-
+ e- →
[Fe(CN)
6
]
4-
is 0.36V. Calculate and report the expected cell potential that the LabQuest meter would give for this cell at 25°C. Show your work and enter your answer in the box. 4.3 (4 pts) Which cell is the cathode? Circle your answer. (
Left cell
or Right cell
) 5. (30 pts total) The following procedure is performed in order to study the kinetics of the target reaction: S
2
O
8
2
-
(aq) + 3 I
‾
(aq) →
2 SO
4
2
-
(aq) + I
3
‾
(aq) Another reaction is coupled with the target reaction to serve as a ‘clock’: 2 S
2
O
3
2
-
(aq) + I
3
‾
(aq) →
S
4
O
6
2
-
(aq) + 3 I
‾
(aq) A student prepared a 0.1482 M standard KI solution. To a test tube containing 0.75 mL of this solution, the student added 0.75 mL of 0.0070 M Na
2
S
2
O
3
and 0.75 mL of KCl with starch. They then added 0.75 mL of 0.200 M Na
2
S
2
O
8
and began timing. The solution turned blue after 127 seconds. 5.1 (6 pts) In the table below, enter the concentration of each reagent in the test tube after the addition of Na
2
S
2
O
8
but before the reaction occurred. Reagents Molarity (M)
I
−
S
2
O
3
2
−
S
2
O
8
2
−
V
Name (PRINT) _________________________________________ Student ID ______________________ 8 5.2 (6 pts) Given that the target reaction is first order in both reactants, what is the rate constant (k) for the reaction? Enter your answer with correct units in the box provided. Show all work to support your answer. 5.3 (6 pts) If the target reaction were second order in iodide and first order in persulfate (it isn’t, but let’s pretend), what would the rate constant (k) for the reaction be? Enter your answer with correct units in the box provided. Show all work to support your answer. 5.4 (12 pts) For each of the changes listed below, and using the correct rate law – first order in both reactants - predict whether it will cause the blue color to appear sooner than, later than, or at 127 seconds. Also indicate whether it will make the initial rate of the target reaction faster, slower, or the same. Circle the correct response. a. 0.75 mL of 0.300M Na2S2O8 is used instead of 0.200 M. The blue color will appear ( sooner than / later than / at ) 127 seconds. The initial rate of the reaction is ( faster / slower / unaffected ). b. 0.75 mL of 0.0060 M Na2S2O3 is used instead of 0.0070 M. The blue color will appear ( sooner than / later than / at ) 127 seconds. The initial rate of the reaction is ( faster / slower / unaffected ).
Name (PRINT) _________________________________________ Student ID ______________________ 9 6. (15 points total) For each amount of substance and piece of equipment paired below, calculate the maximum anticipated relative error in the measurement and enter the value as a percent in the provided box. You must show all of your work and report answers with the correct number of significant figures. a) 24.1 mL delivered from a 10-mL graduated cylinder b) 138.207 g CoCl
2
• 6H
2
O c) 33.2 mL 0.14 M NaOH delivered from a 50-mL Mohr pipet % % %
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