Exam 5: General Chemistry

Passage Potassium permanganate (KMnO₄) is a highly reactive inorganic salt that appears as granular dark purple crystals, nearly black in color.  Due to its useful chemical properties and the fact that its reactions typically do not produce compounds that are toxic, KMnO₄ is employed in a variety of applications, including use as a topical skin disinfectant, an antifungal agent, a fire starter, and a disinfectant for drinking water.KMnO₄ is widely used as an alternative water treatment agent both to disinfect drinking water and to remove undesirable dissolved iron and sulfur compounds.  The crystals are highly soluble in water and form bright pink aqueous solutions that provide a visual indication of the presence of KMnO₄ treatment residual.  As a drinking water disinfectant, a KMnO₄ concentration of 1 mg/L is recommended unless higher levels of contamination are indicated by the absence of a pink residual.Industrial production of potassium permanganate is accomplished in two steps.  In step 1, manganese(IV) oxide (MnO₂) is heated with potassium hydroxide in the presence of oxygen to produce potassium manganate (K₂MnO₄), as outlined in Reaction 1.2 MnO₂ + 4 KOH + O₂ → 2 K₂MnO₄ + 2 H₂OReaction 1In step 2, the potassium manganate is then converted into potassium permanganate electrochemically according to Reaction 2.2 K₂MnO₄ + 2 H₂O → 2 KMnO₄ + 2 KOH + H₂Reaction 2 -In Reaction 1 shown in the passage, the oxidizing agent is:

What is the approximate molar concentration of Na⁺ ions in a solution prepared by dissolving 8.2 g of Na₃PO₄ (MM = 163.94 g/mol) into 250 mL of water?

Passage The primary structure of a protein is a series of amino acids that are connected in a long chain via C−N peptide bonds (Figure 1). Figure 1  Peptide bonds within a protein segment (Note:  Rⁿ = amino acid side chains)Protein chain folding is guided by multiple intramolecular interactions that can occur between two peptide bonds, between a peptide bond and a side chain, or between two side chains, as shown in the protein in Figure 2.  Although these interactions are intramolecular because they are within the same molecule, they act effectively like intermolecular interactions because proteins are such large molecules. Figure 2  Intramolecular interactions between the peptide chain and amino acid side chains within a hypothetical proteinBecause intramolecular interactions play a significant role in maintaining the three-dimensional folded structure of a protein, having a greater number of interactions or stronger interactions will result in increased protein stability.  The energy required to break these interactions within a protein is related to free energy, ΔG.  The greater the ΔG value, the more stable the protein.To find the overall contribution of particular intramolecular interactions to protein stability, researchers made several protein variants in which one amino acid is replaced by another to change the noncovalent interactions between the side chains.  The difference in free energy between the variant and the original protein, Δ(ΔG), indicates the change in overall stability of the protein.  A negative Δ(ΔG) means the variant protein is less stable than the original.Table 1 shows the results of the amino acid substitution experiments for a protein with the approximate energy difference when each amino acid is changed.Table 1  Change in Free Energy Based on Changing Intramolecular Interactions Caused by Amino Acid Substitutions Adapted from: C. N. Pace, J. M. Scholtz, G. R. Grimsley, "Forces stabilizing proteins." FEBS Lett. ©2014 Wiley. -What region of the protein in Figure 2 depicts the weakest type of interaction?

Consider the dimerization of nitrogen dioxide at room temperature: 2 NO2g⇌ N2O4g  K_eq = 8 atm^-1If the equilibrium partial pressure of nitrogen dioxide in a container is 0.5 atm, what is the partial pressure of dinitrogen tetroxide?

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. -The reference electrode for E2 should be maintained at a known, stable concentration of:

Passage Although acetylsalicylic acid (aspirin) is a well-known and widely used analgesic and anti-inflammatory agent, copper(II) acetylsalicylate, a copper complex formed from aspirin and copper, has also been found to exhibit some distinctive and potentially beneficial pharmacological properties.  Formation of copper(II) acetylsalicylate is achieved when the conjugate base of aspirin, acetylsalicylate (C₉H₇O₄⁻), binds to Cu²⁺ ions in solution, as outlined in Reaction 1.4 NaC₉H₇O₄(aq) + 2 CuSO₄(aq) → Cu₂(C₉H₇O₄)₄(s) + 2 Na₂SO₄(aq)Reaction 1Using Reaction 1, a researcher synthesized a sample of copper(II) acetylsalicylate for analysis.  A small volume of 0.50 M aqueous copper(II) sulfate (molar mass = 159.6) was mixed into 100 mL of filtered, saturated aqueous sodium acetylsalicylate (molar mass = 202.2) at room temperature and slowly stirred.  Formation of copper(II) acetylsalicylate (molar mass = 843.7) was immediately observed as a bright blue precipitate.  The precipitate was then isolated by filtration and dried, affording the product complex in high yield. Adapted from: Seguel GV, Rivas BL, Contreras JG. Synthesis and characterization of copper complexes: interaction of metalliccarboxylates with 1,3-thiazole ligand. Journal of the Chilean Chemical Society. 1970; 58(4):1967. -If the solubility of sodium acetylsalicylate is roughly 120 g per 100 mL of water, what is the approximate molar concentration of the saturated NaC₉H₇O₄ solution used in the reaction?

Manganese-54 can undergo a β⁻ decay that is followed by two positron emissions (β⁺ decays), or it can undergo a gamma decay (γ) that is followed by electron capture ε (a form of β⁺ decay).  Which of the following diagrams correctly describes the nuclear decay chain of ⁵⁴Mn?

The diagram below shows the activity of cerium oxides that have been proposed as catalytic materials to reduce emissions of nitrogen monoxide, carbon monoxide, soot, and hydrocarbons during the combustion of diesel fuel.  Based on the diagram, if the reactions are irreversible, which of the following does NOT illustrate a function of the cerium oxide catalyst?

Which of the following is a combustion reaction?

The regulation of mineral ions in the cellular fluids of biological systems is performed by ion pumps that selectively transport ions of specific size and charge across cellular membranes.  Which of the following isoelectronic species is the smallest ion?

Passage Cyanide (CN⁻) is a powerful, rapid toxin that binds to the cellular respiratory enzyme cytochrome c oxidase.  The resulting enzyme-cyanide complex blocks the transfer of electrons to oxygen in the last step of the electron transport chain.  This results in cytotoxic anoxia, causing asphyxiation at the cellular level.Exposure to cyanide can occur from dietary sources, medical treatments, industrial applications, smoke inhalation (structural fires), and intentional use (suicide).  While dietary exposure from moderate consumption of naturally occurring plant compounds such as cyanogenic glycosides rarely results in acute adverse effects in humans, chronic effects have been observed from prolonged diets with significant levels of these compounds.  In the developed world, risk of significant cyanide exposure is most likely to result from smoke inhalation during building fires or from industrial/occupational exposures.The impact of a given exposure depends on the amount (dose, duration), the chemical composition of the cyanide compound (HCN, NaCN, KCN), and the route of exposure (inhalation, absorption, ingestion).  The ionic salts (NaCN, KCN) and free acid (HCN) forms of cyanide each have different acid-base properties that interact differently under the physiological conditions associated with each route of exposure.To better understand the toxicity of different forms of cyanide, animal models have been studied for comparison to known human exposure case studies.  Table 1 presents a summary of selected animal toxicity studies that examine exposure via ingestion.  The table provides the lethal dose to 50% of the population (LD₅₀) in milligrams of cyanide per kilogram of body weight (mg/kg).Table 1  Compilation of LD₅₀ Cyanide Values in Various Animal Species Based on animal models, one treatment developed for cyanide poisoning involves assisting the rhodanese enzyme in metabolizing CN⁻ to SCN⁻ via an intravenous solution of NaNO₂ and Na₂S₂O₃.  The Na₂S₂O₃ assists in the formation of a reactive S-SH (persulfide) bond in the enzyme on cysteine-247 (Reaction 1), which then reacts with the cyanide ion to form thiocyanate (Reaction 2). Adapted from: Hall AH, Isom GE, Rockwood GA. Toxicology of Cyanides and Cyanogens, Experimental, Applied and Clinical Aspects. John Wiley & Sons; 2015: 1-20; and Egekeze JO, Oehme FW. Cyanides and their toxicity: A literature review. Vet Q. 1980; 2(2): 104-114. -In an aqueous solution of 1 × 10⁻³ M HCN (K_a ≈ 1 × 10⁻⁹), HCN can best be described as a:

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. -Which of the following is the correct Lewis dot structure for the common inorganic sunscreen compound titanium dioxide (TiO₂) given that titanium has four valence electrons?

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. -Magnesium corrodes quickly in the body because Mg²⁺ combines with anions other than OH⁻ to form soluble salts.  Based on the solubility data given in Table 1, which of the following salts, if present, will cause the magnesium implant to corrode most quickly?

The bonds of four salts (MgBr₂, NaCl, MgCl₂, and NaBr) are evaluated for ionic character based on electronegativity differences between atoms.  What is the expected order of these four salts if listed according to bond character from the least ionic character to the greatest ionic character?

Passage Copper plays a vital role as a trace mineral for biological processes.  The amount of dietary copper needed by an adult each day is about 30 μg/kg of body weight, and blood plasma normally has a copper concentration of 1 mg/L.Copper is an essential element, and toxic effects are not common.  However, given sufficient exposure to high levels by inhalation, absorption, or ingestion from sources such as industrial hazards or contaminated groundwater, copper toxicity can occur.Salts that are more soluble tend to exhibit higher toxicity when ingested, but the solubilities of copper salts vary and are also temperature-dependent, as shown by selected examples in Figure 1. Figure 1  The solubility of selected copper salts in distilled water with increasing temperatureSome copper salts have solubilities much lower than those shown in Figure 1.  For example, CuF₂ has a solubility of only 0.075 g per 100 g of water (at 25 °C).  This indicates that CuF₂ dissociates in water only to a small extent (Reaction 1) and that the equilibrium favors the undissociated salt, giving CuF₂ a solubility product constant of K_sp = 1.6 × 10⁻⁶. Reaction 1Compound-specific properties can also impact solubility.  In CuF₂, the dissociated F⁻ ion is mildly basic (pK_b = 10.8), causing the equilibrium to be influenced by the pH of the solution.Specific characteristics of copper can also play a role.  For example, copper ions in solution express an affinity to form coordinate bonds with available lone-pair electrons of nitrogen atoms in amines (Reaction 2). Reaction 2If excess amine ligands are present, coordinate bonds like that shown in Reaction 2 will continue to form until each Cu²⁺ ion in solution is coordinated with four nitrogen atoms.  This acquisition of copper ions by ligands in solution can also impact the solubility equilibrium of copper salts by forming soluble coordination complexes. -If excess methylamine (CH₃NH₂) is added to a saturated aqueous CuF₂ solution, the amount of CuF₂ that dissolves will:

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 -What is the molar concentration of Cl⁻ ions in a solution prepared by dissolving 0.1 moles of lanthanum(III) chloride in 1 L of water?

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 citric acid (H₃C₆H₅O₇) is titrated with 0.1 M NaOH to form a citrate buffer solution with a pH of 4.5.  What is the pH at the first equivalence point?  (Note: pK_a1 = 3.13, pK_a2 = 4.76, pK_a3 = 6.40)