Exam 5: General Chemistry

Given that acetic acid (CH₃COOH) has pK_a ≈ 5, the estimated pH of an aqueous 0.001 M sodium acetate (CH₃COONa) salt solution is:

Passage The bicarbonate (HCO₃⁻) buffer system (Reaction 1) helps to maintain acid-base homeostasis and a blood pH near 7.4.  Therefore, concentrations of carbon dioxide (CO₂) and HCO₃⁻ must be tightly regulated through adaptations in respiratory and renal physiology.  For patients with suspected acid-base imbalance, these concentrations can be monitored by blood gas analysis performed on arterial or venous blood. CO2g+H2Ol⇔carbonic anhydraseH2CO3aq⇔pKa=6.35HCO3-aq+H+aqReaction 1In most automated blood gas analyzers, ionized hydrogen, CO₂, and oxygen (O₂) in the sample diffuse through semipermeable membranes and are measured at separate electrodes (Figure 1).  The pH-sensitive glass electrode (E1) is separated from the blood sample by a membrane permeable to hydrogen ions. The sample pH is determined according to the voltage difference between E1 and a reference electrode maintained in a solution of standard pH.  CO₂ diffuses across a gas-permeable membrane to E2, where it reacts to generate HCO₃⁻ and free hydrogen ions.  In a modification of the mechanism used in E1, partial pressure of CO₂ (PaCO₂) is calculated indirectly from the change in pH, as determined by the potential difference between E2 and its reference electrode.E3, also called the Clark electrode, is an electrochemical cell containing a silver anode and a platinum cathode.  O₂ diffuses through another gas-permeable membrane and reacts with hydrogen ions to form water.  The current measured at E3 is then used to calculate PaO₂ in the blood sample. Figure 1  Automated blood gas analyzerThe concentration of carbonic acid (H₂CO₃), reported in milliequivalents per liter (mEq/L), can be calculated from PaCO₂ by Equation 1: H2CO3mEqL=0.03×PaCO2mm HgEquation 1Subsequently, using the concentration of H₂CO₃ obtained from Equation 1, the HCO₃⁻ concentration resulting from decomposition of carbonic acid can then be calculated by applying the Henderson-Hasselbalch equation. -In the Clark electrode, O₂ from the blood is:

Phosphorus-32 is a radioactive beta-emitter used in the treatment of polycythemia vera (excess red blood cells).  The radioactive decay profile for the number of ³²P atoms in a 2.0-mL sample was plotted, as shown below. Based on the graph, what is the concentration of ³²P in the sample after 1.5 half-lives have passed?

Passage An automated external defibrillator (AED) is a medical device used to send an electric shock to the heart after cardiac arrest.  A key component of the AED is the power source, or battery.  Batteries used in AEDs need to have a good charge-to-weight ratio; they must be safe and reliable as well as rechargeable.Two types of rechargeable batteries used in early models of AEDs are the lead storage (also called lead-acid) battery and the nickel-cadmium (NiCd) battery.  These batteries consist of multiple electrochemical cells that are connected in series to deliver a potential between 9 V and 18 V.  A capacitor allows the AED to accumulate charge so that it can deliver between 300 V and 1,000 V.Some AEDs use a sealed lead storage battery.  Lead storage batteries are robust and hold a charge for a long time.  However, they have a low energy-to-weight ratio.  Each lead storage cell delivers approximately 2.0 V, and a battery of four cells weighing 1000 g can provide 30 W∙h of energy. Figure 1  Electron flow in a lead storage battery when discharging and chargingThe half reactions for the anode and cathode of a lead storage battery in 4 M of sulfuric acid (H₂SO₄) are shown in Reactions 1 and 2: Reaction 1 Reaction 2 NiCd batteries have a higher energy-to-weight ratio than lead storage batteries but cannot hold as much charge.  Each NiCd cell delivers approximately 1.3 V, and a single-cell battery weighing 120 g can provide 7.2 W∙h of energy.The half reactions at the anode and cathode for a NiCd battery in KOH are shown in Reactions 3 and 4: Reaction 3 Reaction 4 -If energy density is the amount of energy per unit mass, which of the following statements must be true?

Which gas would occupy more volume at a constant temperature and pressure, 1.5 g of N₂ gas or 1.5 g of O₂ gas?

Passage Heme is found in several proteins, including hemoglobin in blood and myoglobin in muscle.  It has a four-coordinate iron-bound porphyrin structure where ferrous iron (Fe²⁺) is bound to four nitrogen atoms through coordinate covalent bonds.  The most common heme found in the body is heme b in hemoglobin (Figure 1). Figure 1  Structure of heme b found in hemoglobinAlong with the four nitrogen ligands in the porphyrin structure, His F8 also binds to Fe²⁺ in the axial position, leaving the other axial position available to bind oxygen.  In this setting, iron's atomic d orbitals are no longer degenerate.  The energetic difference between the d orbitals depends on the ligands.In deoxygenated heme, iron sits out of the plane formed by the nitrogen ligands in the porphyrin because its d orbitals make it slightly too large to fit in the porphyrin ring.  However, when O₂ binds to heme, iron's electrons re-arrange themselves.  The d orbitals become smaller, and iron sits within the plane of the nitrogen atoms.When iron is scarce in the body, Zn²⁺ will coordinate to the ligands in the protoporphyrin structure through a nonenzymatic process, forming Zn-protoporphyrin IX (ZPP).  ZPP and protoporphyrin IX (PPIX) have distinct emission spectra when excited in the blue range whereas heme does not.  Hematofluorometry can be used as a simple, inexpensive indicator for iron deficiency in whole blood by placing a blood sample in a fluorometer and measuring the amount of ZPP relative to the amount of heme.  Figure 2 illustrates the fluorescence emission spectra for ZPP and protoporphyrin IX measured from a blood sample when ZPP is excited at 425 nm and PPIX is excited at 407 nm. Figure 2  ZPP and PPIX fluorescence emission spectra Adapted from Hennig G, Gruber C, Vogeser M, et al. Dual-wavelength excitation for fluorescence-based quantification of zinc protoporphyrin IX and protoporphyrin IX in whole blood. J Biophotonics. 2014;7(7):514-24. -The figure below shows Zn²⁺ in place of Fe²⁺ to form ZPP, which cannot bind to His F8 or O₂.  Based on the structure of ZPP, which of the following is the coordination number for Zn²⁺?

Passage Students conducted an experiment to determine the changes in enthalpy ΔH°, entropy ΔS°, and Gibbs free energy ΔG° resulting from the dissociation of acetic acid (CH₃COOH) in water.  The students first determined the acid dissociation constant K_a for the reaction by measuring the pH of a 1 M acetic acid solution at various temperatures over a narrow range of less than 10 °C.To perform the pH measurements, the acetic acid solution was initially cooled to 0 °C using an ice bath and then warmed with gentle heating to room temperature with continuous stirring.  The pH was recorded at intervals of 1 °C.  From the pH data, the value of K_a at each temperature T was calculated.  The experimental results over the selected temperature range are shown in Table 1.Table 1  Student Data for the pH and K_a of Acetic Acid with Increasing Temperature To evaluate ΔH° and ΔS°, the students constructed a van 't Hoff plot (Figure 1), which is a graph of the natural log of the dissociation constant (ln K_a) vs. the reciprocal absolute temperature (1/T). Figure 1  Linear van 't Hoff plot relating the dissociation constant K_a of acetic acid to temperature TThe relationship between K_a, T, ΔH°, and ΔS° in the van 't Hoff plot was then described using a linear fit of the data following the form of Equation 1, in which R is the gas constant (8.3 J∙mol⁻¹∙K⁻¹). Adapted from: C. Rezsnyak "Determination of Thermodynamic Values (S°, H°, and G°) from the Dissociation of a Weak Acid" World Journal of Chemical Education. ©2017 Science and Education Publishing. -The slope of the trendline for the data in Figure 1 is −4911 and its y-intercept is 5.4.  Which of the following is an expression for ΔG° at temperature T stated in terms of the data?

Passage Oxytocin is a naturally occurring peptide hormone that is used clinically to prevent postpartum hemorrhage, one of the leading causes of maternal mortality globally.  Because oxytocin is heat and light sensitive, it requires refrigeration at temperatures below 8 °C.  However, in underdeveloped regions with hot climates (such as sub-Saharan Africa), refrigeration is often unavailable for pharmaceutical storage and transport.In an effort to improve the availability of oxytocin in regions with limited refrigeration, researchers conducted two experiments to investigate the use of buffers and divalent alkaline-earth metal salt additives as a means to produce an aqueous oxytocin formulation that is stable when stored at temperatures up to 50 °C.Experiment 1To evaluate effects from buffer concentration, two sets of solutions were prepared containing 0.3 mg/mL oxytocin monoacetate salt in 5, 10, and 50 mM citrate buffer solution made from a mixture of citric acid (a weak acid) and sodium citrate (a salt of the conjugate base).  An unbuffered aqueous solution served as a control sample.  Using 1.0 M NaOH, the buffered solutions were adjusted to pH 4.5 (the pH at which oxytocin is more stable) and then stored at 5 °C (set 1) and 50 °C (set 2) for 4 weeks in a dark area.  After storage, the amount of undegraded oxytocin in each sample was determined (Figure 1) using reverse-phase, high-performance liquid chromatography (RP-HPLC). Figure 1  Oxytocin recovery in a citrate buffer (pH 4.5) and water (pH 6.8)Experiment 2To examine the effect of divalent alkaline-earth metal cation additives, Experiment 1 was repeated, but the solutions were modified to contain 0.3 mg/mL of oxytocin with 5, 10, or 50 mM of a divalent cation (from a chloride salt) dissolved in 10 mM citrate buffer.  The results are shown in Figure 2. Figure 2  Oxytocin in 10 mM citrate buffer with added Ca²⁺, Mg²⁺, or Zn²⁺ salts -Suppose that Experiment 2 is repeated using chloride salts of the Group 1 metals.  Compared with the concentration of Cl⁻ in the 10 mM Mg²⁺ solution, the concentration of Cl⁻ in the 5 mM Na⁺ solution would be:

Passage Depending on the active compound's specific method of action, sunscreens are broadly classified into two groups: organic and inorganic.  Organic sunscreens typically contain aromatic compounds that absorb ultraviolet (UV) radiation. The absorbed energy from the UV radiation causes these compounds to enter electronically excited states which then relax back to the ground state and harmlessly re-emit the absorbed radiation as heat.  Alternatively, inorganic sunscreens contain metallic nanoparticles that serve to reflect, scatter, and absorb incident radiation.  The mechanism for UV protection of each group is shown in Figure 1. Figure 1  Method of action for organic and inorganic sunscreensThe effectiveness of sunscreens is traditionally evaluated based on their ability to prevent reddening of the skin, as indicated by their sun protection factor (SPF).  However, studies have shown that additional deleterious effects, such as the suppression of T cell-mediated immune responses, can occur at lower UV doses than erythema, indicating the need for new metrics to judge sunscreen efficacy.  For this reason, researchers conducted a study to compare the protective ability of six sunscreens against photo-isomerization of the photoreceptor trans-urocanic acid (UCA) to its cis form, a process believed to be linked to UV-induced immunosuppression.  Table 1 lists the sunscreens tested and their relevant characteristics.Table 1  Results from Tested Sunscreens In vivo experiments were conducted on two groups of human volunteers.  Participants in the acute exposure group received a single UV dose of 100 mJ/cm² whereas participants in the chronic exposure group received the same dosage once daily over four consecutive days.  Each sunscreen was applied to a different spot on the volunteers' back prior to irradiation.  A portion of skin without sunscreen was also irradiated to serve as a control.  After irradiation, the sunscreens were removed and the skin was wiped with filter papers.  Filters were washed with potassium hydroxide, and the aqueous solution was analyzed for UCA using high-pressure liquid chromatography.  Figure 2 shows the mean percentages of cis-UCA formed upon irradiation for each exposure type and sunscreen tested. Figure 2  Percentage of UCA in cis configuration upon UV exposure with various sunscreens Adapted from Van der molen RG, Out-luiting C, Driller H, Claas FH, Koerten HK, Mommaas AM. Broad-spectrum sunscreens offer protection against urocanic acid photoisomerization by artificial ultraviolet radiation in human skin. J Invest Dermatol. 2000;115(3):421-6. -Based on the results from the experiment in Figure 2, which of the following conclusions can be drawn?

Passage A group of students were given water from two reservoirs and asked to determine water hardness using flame atomic absorbance (AA) spectroscopy.  Water hardness can be found by relating the concentration of calcium and magnesium ions to the concentration of carbonate species in the sample using Equation 1.[CO₃^2-] = 2.5[Ca²⁺] + 4.1[Mg²⁺]Equation 1To correlate the concentration of the metal ions to absorbance, the students made standard solutions of MgO (MW = 40.31 g/mol) and CaO (MW = 56.08 g/mol) by diluting small portions of 0.01 M MgO and 0.01 M CaO stock solutions with 2% HNO₃ into 100 mL volumetric flasks.  The calibration curves generated from the standard solutions are shown in Figures 1 and 2. Figure 1  Calibration curve from the absorption of the Ca²⁺ solution standards Figure 2  Calibration curve from the absorption of the Mg²⁺ solution standardsLanthanum(III) chloride (MM = 245.25) can be used to eliminate chemical interference in a sample by competitively binding to PO₄³⁻, releasing calcium to be atomized in the flame for detection.The students prepared unknown Samples 1 and 2 by mixing 25 mL reservoir water, 16 mL of 2% HNO₃, and 9 mL of 0.1 M LaCl₃ for a total sample volume of 50 mL each.  The samples were each tested three times using flame AA spectroscopy, yielding the results reported in Table 1.Table 1  AA Spectroscopy Results for Reservoir Water Samples -The "hardness" of a water source is based on the concentration of CO₃²⁻ species.  Water is considered to be "very hard water" if [CO₃²⁻] is at least 0.002 M.  After preparation, is the diluted Sample 2 considered "very hard water"?

Passage Bone growth and remodeling during fracture healing are often slow and have potentially poor outcomes, including malunion or nonunion at the fracture site.  Magnesium bone implants can aid in bone formation and safely biodegrade in the body according to the following reaction:Mg(s) + 2H₂O(l) → Mg(OH)₂(s) + H₂(g)Reaction 1  Corrosion of magnesium metal in aqueous mediumHowever, magnesium metal tends to degrade too quickly and forms hydrogen gas pockets in the body.  Corrosion is slowed as the metal naturally forms a protective layer of magnesium hydroxide in a process called passivation.  Magnesium hydroxide is less soluble than magnesium metal, and its dissolution is described by the endothermic reaction: Reaction 2  Solubility equilibrium of Mg(OH)₂ in water at 25°CAlthough passivation slows down the corrosion rate, it must be slowed even further to adequately facilitate bone reformation.Researchers are interested in the differences in the corrosion rates of magnesium metal protected by different biocompatible coatings.  They compared 10-mm magnesium disks coated with compounds of various cation and anion combinations, with solubility results shown in Table 1.  The coated disks were immersed in simulated body fluid (SBF) at room temperature for 50 hours.Table 1  Solubility for Biocompatible Inorganic Salts Given in mmol/100mL of Pure Water at 25°C Adapted from Huan ZG, Leeflang MA, Zhou J, Fratila-apachitei LE, Duszczyk J. In vitro degradation behavior and cytocompatibility of Mg-Zn-Zr alloys. J Mater Sci Mater Med. 2010;21(9):2623-35. -If researchers wanted to replicate physiological conditions by increasing the SBF temperature to 37°C, how would this change affect the solubility product constant and equilibrium pH of Mg(OH)₂ dissolution?

Passage Heme is found in several proteins, including hemoglobin in blood and myoglobin in muscle.  It has a four-coordinate iron-bound porphyrin structure where ferrous iron (Fe²⁺) is bound to four nitrogen atoms through coordinate covalent bonds.  The most common heme found in the body is heme b in hemoglobin (Figure 1). Figure 1  Structure of heme b found in hemoglobinAlong with the four nitrogen ligands in the porphyrin structure, His F8 also binds to Fe²⁺ in the axial position, leaving the other axial position available to bind oxygen.  In this setting, iron's atomic d orbitals are no longer degenerate.  The energetic difference between the d orbitals depends on the ligands.In deoxygenated heme, iron sits out of the plane formed by the nitrogen ligands in the porphyrin because its d orbitals make it slightly too large to fit in the porphyrin ring.  However, when O₂ binds to heme, iron's electrons re-arrange themselves.  The d orbitals become smaller, and iron sits within the plane of the nitrogen atoms.When iron is scarce in the body, Zn²⁺ will coordinate to the ligands in the protoporphyrin structure through a nonenzymatic process, forming Zn-protoporphyrin IX (ZPP).  ZPP and protoporphyrin IX (PPIX) have distinct emission spectra when excited in the blue range whereas heme does not.  Hematofluorometry can be used as a simple, inexpensive indicator for iron deficiency in whole blood by placing a blood sample in a fluorometer and measuring the amount of ZPP relative to the amount of heme.  Figure 2 illustrates the fluorescence emission spectra for ZPP and protoporphyrin IX measured from a blood sample when ZPP is excited at 425 nm and PPIX is excited at 407 nm. Figure 2  ZPP and PPIX fluorescence emission spectra Adapted from Hennig G, Gruber C, Vogeser M, et al. Dual-wavelength excitation for fluorescence-based quantification of zinc protoporphyrin IX and protoporphyrin IX in whole blood. J Biophotonics. 2014;7(7):514-24. -Which of the following depicts iron's coordination geometry when oxygen is bound to heme b in hemoglobin?

Passage Hyperbaric oxygenation therapy involves placing a patient in a chamber with oxygen-rich air at greater than atmospheric pressure.  Normal air has approximately 78% N₂, 21% O₂, and trace amounts of other gases.  The pressure within the hyperbaric chamber is typically measured in terms of ATA, or atmospheres absolute (1 ATA is the average atmospheric pressure at sea level, or 1 atm).  A hyperbaric chamber set at 2.5 ATA would be the average atmospheric pressure at sea level (1 ATA) plus the additional pressure applied to the chamber (1.5 ATA).There are two main types of hyperbaric chambers: monoplace chambers and multiplace chambers.  Monoplace chambers accommodate a single person.  The air in these chambers is 100% oxygen and can be raised to a maximum pressure of 3 ATA.  Monoplace chambers are used as therapy for conditions where oxygen supply has been depleted, such as carbon monoxide poisoning.  They are also used when a higher concentration of oxygen in plasma is needed, such as in wound healing.Multiplace chambers are large enough to house at least four people.  The air in these chambers is typically a mixture of oxygen and nitrogen, but the patient breathes pure oxygen through a mask.  Some multiplace chambers can endure maximum pressures of 6 ATA and are typically used to eliminate intravascular air bubbles that can occur from decompression sickness or embolisms.  The high pressure from the multiplace chamber results in a decrease in volume of any air pockets within the body, including the sinuses, inner ear, and unwanted air bubbles in the arteries or veins.Patients report that their ears "pop" during the pressurization process, similar to being on an airplane in flight.  In addition, they report that although there is an initial sense of temperature increasing, the chamber's temperature stays relatively stable during both the pressurization and the depressurization processes. -The graph below shows how much certain real gases deviate from ideal behavior.  If the ideal gas law is considered to give a close approximation for the behavior of a real gas that has a deviation of 1.0% or less, based on the graph, which of the following statements is true?

Passage The bicarbonate (HCO₃⁻) buffer system (Reaction 1) helps to maintain acid-base homeostasis and a blood pH near 7.4.  Therefore, concentrations of carbon dioxide (CO₂) and HCO₃⁻ must be tightly regulated through adaptations in respiratory and renal physiology.  For patients with suspected acid-base imbalance, these concentrations can be monitored by blood gas analysis performed on arterial or venous blood. CO2g+H2Ol⇔carbonic anhydraseH2CO3aq⇔pKa=6.35HCO3-aq+H+aqReaction 1In most automated blood gas analyzers, ionized hydrogen, CO₂, and oxygen (O₂) in the sample diffuse through semipermeable membranes and are measured at separate electrodes (Figure 1).  The pH-sensitive glass electrode (E1) is separated from the blood sample by a membrane permeable to hydrogen ions. The sample pH is determined according to the voltage difference between E1 and a reference electrode maintained in a solution of standard pH.  CO₂ diffuses across a gas-permeable membrane to E2, where it reacts to generate HCO₃⁻ and free hydrogen ions.  In a modification of the mechanism used in E1, partial pressure of CO₂ (PaCO₂) is calculated indirectly from the change in pH, as determined by the potential difference between E2 and its reference electrode.E3, also called the Clark electrode, is an electrochemical cell containing a silver anode and a platinum cathode.  O₂ diffuses through another gas-permeable membrane and reacts with hydrogen ions to form water.  The current measured at E3 is then used to calculate PaO₂ in the blood sample. Figure 1  Automated blood gas analyzerThe concentration of carbonic acid (H₂CO₃), reported in milliequivalents per liter (mEq/L), can be calculated from PaCO₂ by Equation 1: H2CO3mEqL=0.03×PaCO2mm HgEquation 1Subsequently, using the concentration of H₂CO₃ obtained from Equation 1, the HCO₃⁻ concentration resulting from decomposition of carbonic acid can then be calculated by applying the Henderson-Hasselbalch equation. -A 50 mL solution of 0.1 M carbonic acid is titrated with a solution of 0.2 M sodium hydroxide (NaOH).  Which of the following represents the curve for this titration?

Passage Sulfur is the 10th most abundant element on Earth and is an essential element in biological systems.  Sulfur can form a variety of chemical compounds and will react with nearly every element except Au, Pt, Ir, Te, and the noble gases.  A list of several common inorganic compounds of sulfur is given in Table 1.Table 1  Selected Common Inorganic Sulfur Compounds Reduction of elemental sulfur by bacteria during anaerobic digestion produces the toxic, foul-smelling gas H₂S.  Oxidation of elemental sulfur via combustion or volcanic processes yields gases of SO₂ and SO₃.  When dissolved in water, both SO₂ and SO₃ produce acidic solutions, and SO₃ reacts with water to form the acid H₂SO₄.  Neutralization of H₂SO₄ can provide a variety of salts, including Na₂SO₄.Elemental sulfur can also react with halogen gases to form sulfur halides such as the industrially important insulator gas SF₆ and the synthetic reagent S₂Cl₂.  S₂Cl₂ facilitates the formation of other sulfur compounds.  For example, reacting S₂Cl₂ with NH₃ produces the sulfur pnictide S₄N₄. -What is the molecular geometry of hydrogen sulfide?

Passage Amino acids and copper both serve vital roles in many biological processes of living organisms.  Beyond their separate biological roles, however, chemists have also discovered that amino acids are able to function as chelates with Cu²⁺ ions to form blue complexes with potential medicinal applications.  One such complex, copper(II) glycinate, has shown to be effective against some oral diseases.  The reaction to form copper(II) glycinate is shown in Figure 1. Figure 1  Synthesis of two isomers of copper(II) glycinateTwo different isomers can form depending on the reaction conditions.  Mixing hot aqueous glycine (70 °C) with a hot solution of copper(II) acetate dissolved in 50% aqueous ethanol, followed by cooling the mixture on ice, yields a precipitate of only the cis isomer.  If a suspension of the cis isomer is held at boiling for an hour or longer in a solution of glycine and copper(II) acetate, the trans isomer is obtained.Because of the geometric differences between the two isomers, the stretching vibrations of the central Cu-N and Cu-O bonds in each isomer are different.  Symmetric and asymmetric bond stretching give four possible vibrational modes for each isomer, as shown in Figure 2. Figure 2  Vibrational modes of the coordinate bonds in copper(II) glycinateIf the stretching of the bonds during vibration causes a transient, unequal distribution of electron density between the atoms within the bond, an oscillating dipole will form.  Bonds with dipoles that are not cancelled out by another dipole in the molecule will absorb infrared light at particular frequencies (often expressed in wave numbers) and are designated as IR-active.  As a result, the vibrational differences between the two isomers can be analyzed by far-infrared (far-IR) spectroscopy, as shown in Figure 3. Figure 3  Far-IR spectra of the cis and trans isomers of copper(II) glycinateBoth isomers show a skeletal absorbance (unrelated to coordinate bond vibration) near 385 cm⁻¹.  Additional absorbance bands are seen that specifically relate to the vibrational modes of the bonds in each isomer. -Suppose that a small sample of the cis isomer precipitate was heated in an oven at 180 °C for 30 minutes, cooled, and then analyzed by far-IR spectroscopy.  If the spectrum of the thermally treated sample was similar to the spectrum of the cis isomer but displayed no absorbance band at 459 or 281 cm⁻¹, the sample most likely was:

Passage Household cleaners commonly contain either ammonia (NH₃) or bleach (NaOCl) as the principal active ingredient.  Mixing ammonia-based and bleach-based cleaners can be hazardous, as the resulting reactions can form several toxic products.  One of these reactions forms the irritant dichloramine (NHCl₂) as follows: Reaction 1The overall reaction has a standard enthalpy change of −55 kJ/mol and a standard entropy change of −150 J/mol·K.  Partial thermodynamic data for formation of Reaction 1 reagents from their elements are shown in Table 1.Table 1  Partial Data for Enthalpies, Entropies, and Free Energies of Formation of Several Compounds at 25 °C After it is formed, dichloramine readily enters the gas phase, where it may be inhaled.  In the lungs, dichloramine reacts with water to produce hydrochloric acid and ammonia, as well as reactive oxygen species.  In small doses, these products cause mild irritation, but inhalation of large amounts of chloramines can lead to pneumonitis (inflammation of the lungs) and may require medical intervention. Adapted from Trogolo D, Arey JS. Benchmark thermochemistry of chloramines, bromamines, and bromochloramines: halogen oxidants stabilized by electron correlation. Phys Chem Chem Phys. 2015;17(5):3584-98. -Assuming ΔH° and ΔS° are constant at all temperatures, at approximately which temperature (in degrees Celsius) will ΔG° of Reaction 1 equal zero (ie, switch from spontaneous to nonspontaneous)?

Which of the following will occur during the gamma decay of an atom of technetium-99m?The nucleus will emit a high-energy photon.The number of protons and neutrons in the nucleus will be unchanged.The atom will be converted into an isotope of a different element.

If each of the following is an ideal solution at 37 °C, the lowest osmotic pressure, equaling 20 atm, is produced by:

Passage Water is unique in that all three of its phases exist over a small range of pressures and temperatures, as shown in Figure 1. Figure 1  Phase diagram for waterIn the liquid phase, water dissolves and transports molecules, and aids in the biochemical reactions necessary to support life.  For this reason, scientists believe liquid-phase water is essential to the existence of extraterrestrial life.  Although it has been known for some time that water is present in both the gas and ice phases on Mars, the question remains as to whether there are regions where liquid-phase water can stably exist.  Estimates of Mars' surface temperature and pressure generated from satellite readings and global climate models, as well as observations made by Mars rovers, reveal that surface conditions on the planet can vary greatly.Table 1  Temperature and Pressure of Various Locations on Mars Despite the extremely low pressures and temperatures on Mars, satellite images have repeatedly found evidence of flowing liquid water in several regions during the late Martian summer season.  These regions, called recurring slope lineae (RSL), are hypothesized to be thermodynamically stable under certain Martian conditions due to the presence of dissolved salts.  A simple understanding of this hypothesis is afforded by Raoult law, which states that the vapor pressure and freezing point of an ideal solution are lowered as the number of moles of dissolved solute increases.  Moreover, this effect also impacts the solution boiling point (the point at which the vapor pressure of the solution becomes equal to the ambient pressure).The briny-water hypothesis was confirmed in 2015 when spectral data from the Compact Reconnaissance Imaging Spectrometer onboard the Mars Reconnaissance Orbiter revealed the presence of hydrated perchlorates in the same regions and times that RSL were observed.  Studies are already underway to determine how common such briny water flows are and whether the conditions within them are suitable for microbial life. -Which of the following does not explain water's ability to act as a solvent?