QUESTION 1
Topic 4.5 Stoichiometry
(a) TOPIC 1.1 Moles andMolar Mass
(b) TOPIC 1.1Moles andMolar Mass
(c) TOPIC 3.2 Properties of Solids
(d) TOPIC 3.2 Properties of Solids
(e) Topic 8.7 pH and pKa
(f) Topic 8.7 pH and pKa
(g) Topic 8.7 pH and pKa
(h) Topic 8.7 pH and pKa
1. A student reacts 0.300 g of methyl salicylate (C8 H8 O3) with a stoichiometric amount of a strong base. This product is then acidified to produce salicylic acid crystals (HC7 H5 O3 ) .
(a) For every 1 mole of C8 H8 O3 (molar mass 152.15 g/mol) reactant used, 1 mole of salicylic acid crystals (HC7 H5 O3 , molar mass 138.12 g/mol) 3 is produced. Calculate the maximum mass, in grams, of HC7 H5 O3 that could be produced in this reaction.
As part of the experimental procedure to purify the HC7 H5 O3 crystals after the reaction is complete, the crystals are filtered from the reaction mixture, rinsed with distilled water, and dried. Some physical properties of HC7 H5 O3 are given in the following table.
(b) The student’s experiment results in an 87% yield of dry HC7 H5 O3. The student suggests that some of the HC7H5 O3 crystals dissolved in the distilled water during the rinsing step. Is the student’s claim consistent with the calculated percent yield value? Justify your answer.
(c) Given the physical properties in the table, calculate the quantity of heat that must be absorbed to increase the temperature of a 0.105 g sample of dry HC7H5O3 (molar mass 138.12 g / mol) 3 crystals from 25 C° to the melting point of 159°C and melt the crystals completely
The structures and melting points for methyl salicylate and salicylic acid are shown.
(d) The same three types of intermolecular forces (London dispersion forces, dipole-dipole interactions, and hydrogen bonding) exist among molecules of each substance. Explain why the melting point of salicylic acid is higher than that of methyl salicylate.
The student titrates 20.0 mL of 0.0100 M HC7H5O3(aq)with 0.0200 M NaOH, using a probe to monitor the pH of the solution. The data are plotted producing the following titration curve.
(e) Using the information in the graph, estimate the pKa of HC7H5 O3. _______________
(f) When the pH of the titration mixture is 4.00, is there a higher concentration of the weak acid, HC7H5 O3, or its conjugate base, C7H5 O3 , in the flask? Justify your answer.
(g) The student researches benzoic acid (HC7H5O2) and finds that it has similar properties to salicylic acid ( HC7H5 O3). The Ka for benzoic acid is 6.3 * 10−5 . Calculate the value of pKa for benzoic acid.
(h) The student performs a second titration, this time titrating 20.0 mL of a 0.0100 M benzoic acid solution with 0.0200 M NaOH. Sketch the curve that would result from this titration of benzoic acid on the following graph, which already shows the original curve from the titration of 20.0 mL of 0.0100 M salicylic acid. The initial pH of the benzoic acid solution is 3.11.
▶️Answer/Explanation
(a) For the correct calculated value:
0.300g \(C_{8}H_{8}O_{3}\times \frac{1molC_{8}H_{8}O_{3}}{152.15g}\times\frac{1molHC_{7}H_{5}O_{3}}{1molC_{8}H_{8}O_{3}}\times \frac{138.12g}{1molHC_{7}H_{5}O_{3}}=0.272gHC_{7}H_{5}O_{3}\)
(b) For the correct answer and a valid justification:
Yes (consistent). Because the acid is soluble in water, some crystals may dissolve during rinsing, causing the mass of the collected precipitate to be lower than expected. This would lead to a percent yield less than 100%.
(c)For the correct calculated value of the other q and the total heat:
q total = q heat + q melt = 16.5 J + 20.6 J = 37.1 J
(d) For a correct explanation:
Molecules of salicylic acid have more hydrogen bonding sites than molecules of methyl salicylate have, which leads to stronger intermolecular forces and a higher melting point for salicylic acid.
(e) For the correct answer:
The p K a is approximately 3.
(f) For the correct answer and a valid justification, consistent with part (e):
Accept one of the following:
• The conjugate base, − C7 H5O3 . When pH = 4 , the titration is beyond the half equivalence point, where − [HC 7H5O3 ]= [C7 H5O3 ] . Thus, − [C 7H5O3 ] must be greater than [HC7H5O3 ].
• The conjugate base, − C7 H5O3 . Because the pH of the solution is greater than the p Ka of the acid, the majority of the molecules will be deprotonated.
(g) For the correct calculated value:
pKa = −log(6.3 × 10 ) = 4.20
(h) For a curve that shows a correct starting and half-equivalence point, consistent with part (g):
The curve starts at pH ≈ 3.11 and passes through the p K a calculated in part (g) at 5 mL . See example response below.
QUESTION-2
Topic 5.11 Catalysis
(a) Topic 4.2 Net Ionic Equations
(b) TOPIC 2.5 Lewis Diagrams
(c) TOPIC 9.2 Absolute Entropy and Entropy Change
(d) TOPIC 9.5 Free Energy and Equilibrium
(e) TOPIC 9.5 Free Energy and Equilibrium
(f) TOPIC 9.5 Free Energy and Equilibrium
(g) TOPIC 9.5 Free Energy and Equilibrium
\( CH_{3}OH(g)\rightleftharpoons CO(g) + 2 H_{2}(g)\\\ \Delta H^{\circ }= 90.0\\\ kj/mol_{rxn}\)
2. Methanol vapor decomposes to form carbon monoxide gas and hydrogen gas at high temperatures in the presence of a platinum catalyst, as represented by the balanced chemical equation given.
(a) Are the hydrogen atoms oxidized or are they reduced in the forward reaction? Justify your answer in terms of oxidation numbers.
(b) In the following box, draw the complete Lewis electron-dot diagram for the carbon monoxide molecule in which every atom obeys the octet rule. Show all bonding and nonbonding valence electrons.
(c) The values of the standard molar entropies of the compounds involved in the reaction are given in the following table.
(i) Use the data in the table to calculate the value of the standard entropy change, ∆S°, in \(J/\left ( K.mol_{rxn} \right )\), for the reaction.
(ii) Calculate the value of ∆G°, in \(kJ/\left (K.mol_{rxn} \right)\), for the reaction at 375 K. Assume that ∆H° and ∆S° are independent of temperature.
The following particle-level diagram shows a representative sample of the equilibrium mixture represented by the equation given.
(d) Use information from the particle diagram to calculate the partial pressure of CO at equilibrium when the total pressure of the equilibrium mixture is 12.0 atm.
(e) Write the expression for the equilibrium constant, Kp, for the reaction.
CH3OH(g) \(\rightleftharpoons \) CO(g) + 2 H2(g)
The reaction system represented by the equation is allowed to achieve equilibrium at a different temperature. The following table gives the partial pressure of each species in the equilibrium mixture.
(f) Use the information in the table to calculate the value of the equilibrium constant, Kp, at the new temperature.
(g) The volume of the container is rapidly doubled with no change in temperature. As equilibrium is re-established, does the number of moles of CH3OH(g) increase, decrease, or remain the same? Justify your answer by comparing the value of the reaction quotient, Q, with the value of the equilibrium constant, Kp
▶️Answer/Explanation
(a) For the correct answer and a valid justification:
The H atoms are reduced because they change from an oxidation number of +1 to 0.
(b) For the correct answer:
(c)For the correct calculated value:
(i) \(\Delta S^{\circ }_{rxn}=198+2\left ( 131 \right )-240.=220.\frac{J}{K.mol_{rxn}}\)
(ii) For the correct calculated value:
\(\Delta G^{\circ }=\Delta H^{\circ -}T\Delta S^{\circ }\)
\(\Delta G^{\circ }=90.0\frac{kJ}{mol_{rxn}}-\left ( 375K \right )\left ( 0.220\right )\left ( 0.220\frac{kJ}{K.mol_{rxn}} \right )=+7.5kJ/mol_{rxn}\)
(d) For the correct calculated value:
\(P_{co}=\frac{3}{10}\left ( 12.0atm \right )=3.6atm\)
(e) For the correct expression:
\(K_{p}=\frac{\left ( P_{co} \right )\left ( P_{H2} \right )^{2}}{\left ( P_{CH_{3}OH} \right )}\)
(f) For the correct calculated value:
\( K_{p} = \frac{(P_{co})(P_{H_{2}})^{2}}{P_{CH_{3}OH}} = \frac{(4.2(8.4)^{2})}{(2.7)} = 110\)
(g) For a correct comparison of Q and K:
Accept one of the following:
• The change in volume causes the partial pressure of each species to decrease by a factor of two. Because there are more moles of gaseous products than reactants, the decrease of the numerator in Q will be larger than that in the denominator, making Qp < Kp.
For the correct answer and a valid justification:
Decrease. Given that \(Q_{p}<K_{p,}\) the partial pressures (moles) of the products will increase as equilibrium re-establishes, decreasing the number of moles of \(CH_{3}OH\)
QUESTION-3
TOPIC 1.5 Atomic Structure and Electron Configuration
(a) TOPIC 1.5 Atomic Structure and Electron Configuration
(b) TOPIC 1.5Atomic Structure and Electron Configuration
(c) Topic 3.7 Solutions and Mixtures
(d) Topic 3.7 Solutions and Mixtures
(e) TOPIC 9.9 Cell Potential and Free Energy
(f) TOPIC 9.9 Cell Potential and Free Energy
(g) TOPIC 9.9 Cell Potential and Free Energy
3. Answer the following questions relating to the element aluminum, Al.
(a) Write the complete ground-state electron configuration of an Al atom.
(b) Based on principles of atomic structure, explain why the radius of the Al atom is larger than the radius of the \(AI^{3+}\) ion.
A student plans to combine solid aluminum with an aqueous solution of silver ions. The student determines the mass of solid AgNO3 needed to prepare the solution with a specific concentration.
(c) In the following table, briefly list the steps necessary to prepare 200.0 mL of an aqueous solution of AgNO3 using only equipment selected from the choices given. Assume that all appropriate safety measures are already in place. Not all equipment or lines in the table may be needed.
Solid AgNO3 .Weighing paper and scoop · ·250 mL beakers
Distilled water . 200.00 mL volumetric flask Pipet
Balance . 50.0 mL graduated cylinder
After preparing the solution, the student places some of the solution into a beaker and adds a sample of aluminum. The reaction represented by the following equation occurs.
AI \(\left ( s \right )+3Ag^{\dotplus }\left ( aq \right )\to AI^{3^{\dotplus }}\left ( aq \right )+Ag\left ( s \right )\)
(d) The following diagram gives an incomplete particulate representation of the reaction. The beaker on the left represents the system before the mixture reacts. Complete the drawing on the right to represent the system after the reaction has occurred. Be sure to include 1) the correct type and number of particles based on the number shown on the left and 2) the relative spacing to depict the appropriate phases
The student finds the standard reduction potentials given in the table, which are related to the reaction that occurs.
(e) Using the standard reduction potentials, calculate the value of E° for the reaction.
(f) Based on the value of E°, would the standard free energy change of the reaction under standard conditions, ∆G°, be positive, negative, or zero? Justify your answer.
(g) Once the reaction appears to stop progressing, would the change in free energy, ∆G, be positive, negative, or zero? Justify your answer.
▶️Answer/Explanation
(a) For a correct electron configuration:
Accept one of the following:
\(1{s_{}}^{2}2{s_{}}^{2}2{_{p}}^{6}3{s_{}}^{2}3{p_{}}^{1}\)
\(\left [ Ne \right ]3{s_{}}^{2}3{p_{}}^{1}\)
(b) For a correct explanation:
The highest occupied electron shell (n=3) of Al is at a greater average distance from the nucleus than the highest occupied electron shell (n=2) of Al3+
(c) For the correct step to ensure quantitative dilution:
5. After the solid is dissolved, fill the flask to the calibration ( 200.00 mL ) mark and mix.
(d) For a drawing that shows product formation and indicates the conservation of matter:
4 Al and 8 Ag particles in the beaker on right (see sample drawing below)
(e) For the correct calculated value:
Accept one of the following:
(f) For the correct answer and a valid justification:
Negative. The reaction has a positive value of E°, indicating that it is thermodynamically favorable and would therefore have a negative value of ∆ ° G . ( ∆G°=−nFE° )
(g) For the correct answer and a valid justification:
Accept one of the following:
• Zero. The observation that the reaction stops progressing implies that \(E_{Cell} = 0\) , indicating that there is no longer a driving force for the reaction.
• Zero. The observation that reaction stops progressing implies that equilibrium is established, and ∆ = G 0 at equilibrium.
Question-4
TOPIC 2.5 Lewis Diagrams
(a) TOPIC 1.1Moles and Molar Mass
(b) TOPIC 3.1 Intermolecular and Interparticle Forces
(c) TOPIC 3.2 Properties of Solids
4. Answer the following questions about the compounds NH2Cl and NCl3. The Lewis electron-dot diagrams of the two compounds are shown.
(a) Calculate the number of moles of NH2Cl (molar mass 51.48 g/mol) present in 1.0 L of a solution in which the concentration of NH2Cl is 0.0016 g/L.
(b) NH2Cl is highly soluble in water, whereas NCl3 is nearly insoluble. Explain this observation in terms of the types and relative strengths of the intermolecular forces between each of the solutes and water.
(c) The value of \(\Delta H^{\circ} _{vaporization}\) for NCl3(l) is 32.9 kJ/mol. Calculate the amount of energy required to vaporize a 15.0 g sample of NCl3 (molar mass 120.36 g/mol).
▶️Answer/Explanation
(a) For a correct calculated value:
1L\times \frac{0.0016g}{1L}\times \frac{1mol}{51.48g}=3.1\times 10^{-5}mol
(b) For the correct identification of intermolecular forces between each substance and water:
Accept one of the following:
• Both NH_{2}Cl and NCl_{3}can participate in hydrogen bonding with water.
• Both NH_{2}Cl and NCl_{3} have dipole-dipole attractions to water.
(c) For the correct calculated value:
15.0gNCl_{3}\times\frac{1mol}{120.36g}\times \frac{32.9kJ}{1mol}=4.10kJ
QUESTION-5
Topic 5.2 Introduction to Rate Law
(a) Topic 5.3 Concentration Changes Over Time
(b) Topic 5.2 Introduction to Rate Law
(c) Topic 7.7 Calculating Equilibrium Concentrations
5. The following equation represents the decomposition of N2 O5, for which the rate law is rate =\(k\left [ N_{2}O_{5} \right ]\).
2 N2 O5 (g) → 4 NO2 (g) + O2 (g)
A sample of pure N2 O5 (g) is placed in an evacuated container and allowed to decompose at a constant temperature of 300 K. The concentration of N2O5 (g) in the container is measured over a period of time, and the measurements are recorded in the following table.
(a) Determine the value of the rate constant, k , for the reaction. Include units in your answer.
(b) The following mechanism is proposed for the decomposition of N_{2}O_{5}\left ( g \right ).
Identify which step of the proposed mechanism (1, 2, or 3) is the rate-determining step. Justify your answer in terms of the rate law given.
(c) If this experiment was repeated at the same temperature but with twice the initial concentration of N2 O5, would the value of k increase, decrease, or remain the same? Explain your reasoning.
▶️Answer/Explanation
(a) For the correct calculated value:
Accept one of the following:
(b) For the correct answer and a valid justification:
Step 1 is the rate-determining step. The rate law of elementary step 1 is rate = k[N_{2}O_{5}] , which is consistent with the first order kinetics of the overall rate law.
(c) For the correct answer:
Remain the same. The rate constant, k, is independent of concentration and will remain the same at constant temperature.
QUESTION 6
Topic 5.3 Concentration Changes Over Time
(a) Topic 3.12 Properties of Photons
(b) TOPIC 7.1 Introduction to Equilibrium
6. A student wants to determine the concentration of permanganate, MnO4 −( aq), in a solution. The student plans to use colorimetric analysis because solutions containing MnO4−(aq) have a purple color.
(a) To determine the optimum wavelength for an experiment that measures the concentration of MnO4−(aq ), the student takes a sample of the solution and measures the amount of light absorbed by the sample over a range of wavelengths. The data are plotted in the graph shown. Identify the optimum wavelength that the student should use for the experimental procedure.
(b) The student uses a stock solution of \(2.40\times 10^{-3}M KMnO_{4}\left ( aq \right )\) to prepare the standard solutions of \(MnO_{4}\left ( aq \right )\) that are needed to construct a calibration curve.
(i) The student uses a 100.0 mL graduated cylinder to measure a certain volume of KMnO4(aq) stock solution, as shown in the diagram given. What volume should the student record?
(ii) Calculate the volume, in mL, \(of2.40\times 10^{-3}MKMnO_{4}\left ( aq \right )\) that is required to produce 100.0 mL of a standard \(1.68\times 10^{-3}MMnO_{4}^{-\left ( aq \right )}\) solution.
The student designs the following procedure to produce a calibration curve.
Step 1: Prepare several standard solutions that have known MnO4−(aq ) concentrations by dilution of the stock solution.
Step 2: Rinse the cuvette with distilled water.
Step 3: Rinse the cuvette with the standard solution and fill the cuvette with the standard solution.
Step 4: Measure the absorbance of the standard solution with the colorimeter.
Step 5: Repeat steps 2-4 for each of the standard solutions.
The data are plotted in the calibration curve shown. One of the data points (indicated with an arrow) on the calibration curve is below the line of best fit.
(c) Assuming that all lab equipment is functioning properly, identify which one of the procedural steps the student could have executed incorrectly that would explain why the marked data point is below the line of best fit. Justify your answer.
▶️Answer/Explanation
(a) For the correct answer:
525 nm
(b)(i) For the correct answer:
92.0 mL
(ii) For the correct calculated value:
\(V_{1}=\frac{M_{2}V_{2}}{M_{1}}=\frac{\left ( 1.68\times 10^{-3}M \right )\left ( 100.0mL \right )}{2.40\times 10^{-3}M}=70.0mL\)
(c) For the correct answer and a valid justification:
The student could have improperly executed step 3. If the cuvette was not rinsed with the standard solution prior to being filled for the measurement of absorbance, the standard solution would be diluted by the remaining distilled water, and the absorbance would be lower than what it should be.
QUESTION 7
TOPIC 2.5 Lewis Diagrams
(a) TOPIC 2.7 VSEPR and Hybridization
(b) Topic 3.10 Solubility
7. A Lewis electron-dot diagram of the oxalate ion, \(C_{2}O_{4}^{2-}\) , is shown.
(a) Identify the hybridization of the valence orbitals of either carbon atom in the oxalate ion.
(b) Silver oxalate, \(ag_{2}C_{2}O_{4}\left ( s \right )\), is slightly soluble in water. The value of \(K_{sp}forAg_{2}C_{2}O_{4} is 5.40\times 10^{-12}\).
(i) Write the expression for the solubility-product constant, \(K_{sp}forAg_{2}C_{2}O_{4}\)
(ii) Calculate the molar solubility of \(Ag_{2}C_{2}O_{4}\) in neutral distilled water.
(iii) The molar solubility of \(Ag_{2}C_{2}O_{4}\) increases when it is dissolved in \(0.5MHCIO_{4}\left ( aq \right )\) instead of neutral distilled water. Write a balanced, net-ionic equation for the process that occurs between species in solution that contributes to the increased solubility of \(Ag_{2}C_{2}O_{4}\left ( aq \right ) in HCIO_{4}\left ( aq \right )\)
▶️Answer/Explanation
p>(a) For the correct answer:
\(sp^{2}\)
(b)(i) For the correct answer:
\(K_{sp} = [Ag^{+}]^{2}[C_{2}O_{4}^{2-}]\)
(ii) For the correct calculated value:
\(5.40\times 10^{-12}=\left ( 2_{s} \right )^{2}\left ( s \right )\)
\(5.40\times 10^{-12}=\left ( 4_{s} \right )^{3}\)
\(s=1.11\times 10^{-4}M\)
(iii) For a correct equation (state symbols not required):
Accept one of the following: