Exam 5: General Chemistry

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. -Which of the following will increase the amount of CuF₂(s) that dissolves in water and raise the Cu²⁺(aq) concentration in the solution?  (Note: Cu(OH)₂ has a K_sp = 2.2 × 10⁻²⁰.)

In elements 1-86 on the periodic table, one particular group contains one element that forms a diatomic gas in its elemental state, at least one metalloid, two nonmetal solids that are not metalloids, and one element that is exclusively radioactive.  The group is:

Passage Magnetic resonance imaging (MRI) interprets the nuclear relaxation or relaxivity of hydrogen nuclei in resident water molecules.  Hydrogen nuclei in water have two magnetic spin states:  alpha and beta.  In the presence of an applied magnetic field, these spins will align with the field in an "excited" state and then relax back to the "ground" state, producing a signal with intensity proportional to relaxivity.  Frequently, an MRI contrast agent is used to increase the relaxation time in coordinated molecules, resulting in a more intense signal.Commonly used contrast agents include gadolinium-based agents.  Depending on its environment, gadolinium (Gd) can have eight or nine sites in its coordination sphere, allowing Gd³⁺ to interact with eight water molecules as in Gd(H₂O)₈³⁺.  The electronic configuration of gadolinium is shown in Figure 1. Figure 1  Electronic configuration of gadoliniumGd³⁺ alone is toxic to cells because its ionic radius is similar to that of Ca²⁺, allowing Gd³⁺ to displace Ca²⁺ in biologically important settings.  Therefore, Gd³⁺ must be coordinated to an organic ligand such as 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) to allow it to pass through the body safely.  When chelated, DOTA displaces the water around gadolinium and leaves only one coordination site for a water molecule (Figure 2). Figure 2  Gd-DOTA with a water coordinated to Gd³⁺Zinc ions (Zn²⁺) are required to store insulin and are co-released when insulin is secreted by the pancreas.  To detect zinc ions in the body, researchers have designed a variation of Gd-DOTA called Gd-daa3 with two diaminoacetate (daa) arms that preferentially bind to Zn²⁺ ions (Figure 3).  When Zn²⁺ is absent, these arms coordinate to Gd³⁺ and create a nine-coordinate complex that prohibits water from binding to Gd³⁺.  Binding of diaminoacetate arms to Zn²⁺ frees Gd³⁺ to coordinate with water molecules.  This configuration results in increased signal intensity in the parts of the cell where Zn²⁺ is located. Figure 3  Gd-daa3 when Zn²⁺ is present Adapted from Louie A. MRI biosensors: a short primer. J Magn Reson Imaging. 2013;38(3):530-9. -The Heisenberg uncertainty principle impacts the study of nuclei and small particles such as electrons.  Which of the following is a consequence of this principle?

The product of pressure P and volume V of an ideal gas is directly proportional to the product of the moles n and absolute temperature T of the gas, as shown below. What is the value of the gas constant R derived from the graph of the data?

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. -Compared to the boiling point of a sample of pure water on Mars, the boiling point of a sample of briny water under the same atmospheric conditions would be:

Equilibrium is reached in a reversible reaction when the rate of the forward reaction equals the rate of the reverse reaction.  The graph shows the changes in concentration during the formation of HI(g) from a mixture of H₂(g) and I₂(g).  If the measurement is taken at equilibrium, how many moles of HI(g) are present in a 750 mL sample of the reaction mixture?

Which of the following compound pairs, dissolved into solution at equal concentrations, will function as a buffer?

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. -How will the volume of inhaled oxygen at a constant pressure be affected if the ambient temperature in the chamber is 29 °C and body temperature is 37 °C?

An 84-mg sample of a compound is found to contain 36 mg of carbon, 3 mg of hydrogen, 21 mg of nitrogen, and 24 mg of oxygen.  If the compound has a molecular weight of 112 g/mol, what is its molecular formula?

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 -As an alternative to the industrial preparation outlined by Reactions 1 and 2, potassium permanganate can be prepared in the lab by each of the reactions shown below.  All of these can be classified as a disproportionation reaction EXCEPT:2 K₂MnO₄ + Cl₂ → 2 KMnO₄ + 2 KCl3 K₂MnO₄ + 4 HCl → 2 KMnO₄ + MnO₂ + 2 H₂O + 4 KCl3 K₂MnO₄ + 2 CO₂ → 2 KMnO₄ + 2 K₂CO₃ + MnO₂

Atoms of solid polonium are packed in a repeating array that can be represented by a cubic unit cell, which is the simplest three-dimensional cross-section that repeats throughout the solid. If the atomic radius r is 1.67 × 10⁻⁸ cm and the mass of one polonium atom is 3.47 × 10⁻²² g, which expression correctly calculates the density of a pure sample of solid polonium?

A reaction between chloride and permanganate ions occurs in an acidic solution, as shown below:5 Cl⁻(aq) + 2 MnO₄⁻(aq) + 11 H⁺(aq)  →  2 Mn²⁺(aq) + 5 HClO(aq) + 3 H₂O(l)Which element undergoes a change in oxidation state corresponding to a loss of two electrons?

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. -What is the molarity of cinoxate (0.250 kg/mol) in 100 mL of sunscreen B16 (density = 1 g/mL)?

Which statement correctly describes both the 3s and 3p energy levels in the lowest-energy electron configuration of the phosphide anion P³⁻ ?

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 -What volume of H₂ gas will be generated in Reaction 2 described in the passage if 10.00 L of a 5.00 M solution of K₂MnO₄ react at standard temperature and pressure (STP)?

The emission line spectrum of hydrogen displays four spectral lines in the visible region of the spectrum. According to the Bohr model of the atom, which of the diagrams below represents the spectral line at 410 nm in the emission line spectrum?

Passage Kidney stones are a common ailment affecting approximately 10% of adults in the United States.  They form when solutes precipitate out of solution as crystals in the urinary tract, and they can cause severe pain in the side, back, abdomen, and groin.  Individuals who have been previously diagnosed with kidney stones have an increased probability of developing new stones relative to unaffected individuals.  Different measures may help prevent the formation of different kinds of stones, so analysis of the composition of stones that have been passed or removed can aid in preventing recurrence.  Stones can be ground into fine powders, dissolved in a small amount of solvent, and analyzed by infrared (IR) spectroscopy, as shown in Figure 1. Figure 1  Schematic of kidney stone analysis by IR spectroscopyIR analysis of kidney stones from 50 individuals revealed the percentage of stones that contain each component, shown in Table 1 along with solubility data.Table 1  Kidney Stone Composition Parameters Some studies indicate that potassium citrate, taken orally, may prevent the formation of calcium oxalate crystals, the most abundant component of kidney stones.  Oxalic acid, shown in Figure 2, is significantly more soluble than calcium oxalate. Figure 2  Structure of oxalic acid and its associated anions with increasing pHPotassium citrate alkalinizes the urine, potentially causing a decrease in oxalate solubility and the formation of more crystals.  However, potassium citrate can also react with calcium oxalate according to the unbalanced equation shown in Reaction 1:CaC₂O₄ + K₃(C₆H₅O₇) → Ca₃(C₆H₅O₇)₂ + K₂C₂O₄Reaction 1Calcium citrate and potassium oxalate are both hundreds of times more soluble than calcium oxalate, so the presence of citrate and potassium ions can help maintain calcium and oxalate ions in solution.  This effect may be sufficient to overcome the decreased solubility that occurs at higher pH levels. Adapted from Primiano A, Persichilli S, Gambaro G, et al. FT-IR analysis of urinary stones: a helpful tool for clinician comparison with the chemical spot test. Dis Markers. 2014;2014:176165. -Which of the following bond types are found in the calcium phosphate present in 35% of kidney stones?IonicPolar covalentNonpolar covalent

Passage Within the renal system, two buffers operate to counter the production of excess H⁺ from luminal H₂CO₃.  They are located within the tubular lumen, and maintain urinary pH levels between 4.5 and 8.0.The first of these is a phosphate buffer, which serves as the primary buffer for urine because its pK_a is very close to extracellular fluid pH of 7.4.  The phosphate buffer operates according to Reaction 1.  H₂PO₄⁻ is excreted in urine. Reaction 1During acidosis when tubular pH is low, the phosphate buffer system becomes saturated.  An ammonia buffer system then becomes the primary buffer, and NH₄⁺ is excreted in urine. Reaction 2Titratable acid (TA) is the total acid in urine, and it can be found using Folin's method.  Under normal physiological conditions, TA is approximately equal to the amount of phosphate in urine, which is a product of serum phosphate concentration and glomerular filtration rate.  This makes TA a good indicator of kidney function.Folin's method involves back titration of a urine sample to a pH of 7.4 using a strong base.  In a typical procedure, potassium oxalate is added to remove calcium from the sample prior to titration with 1.0 M NaOH.  The titration continues until the solution reaches a pH of 7.4 (measured by a pH meter).  The amount of base required for the titration can then be used to calculate the amount of H₂PO₄⁻ in the urine sample. -The chemical process that occurs during acidosis can be replicated in vitro by adding a strong acid to an ammonia buffer solution.  How will the pH change in the solution if 1.0 mL of 0.5 M H₂SO₄ is added to a 100 mL solution containing 1 M NH₃ and 1 M NH₄⁺?

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. -Blood gas analysis results are listed below.  From the data, determine the HCO₃⁻ concentration in the blood sample resulting from the decomposition of carbonic acid.

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 -How many moles of O₂ are consumed if 1.8 kg of MnO₂ (MM = 86.9 g/mol) are used in Reaction 1?