Exam 5: General Chemistry

Passage The alkaline-earth metals are a highly reactive family of metals occupying Group 2 (IIA) of the periodic table.  In their elemental form, the alkaline-earth metals have a shiny, silvery-white to slightly yellowish appearance.  The alkaline-earth metals exhibit some similarities to the chemistry of the alkali metals (Group 1) but are a little less reactive.Atoms of the alkaline-earth metals (M) have only two valence electrons and a low first and second ionization energy.  Consequently, these metals readily lose both electrons to produce cations with a +2 charge (M⁺²) during reactions with nonmetals.  As a result, the alkaline-earth metals do not exist in their elemental metallic form in nature.  Instead, the Group 2 elements are found primarily as metal oxides, which form by reacting oxygen (Reaction 1). Similarly, elemental samples of alkaline-earth metals will react with elemental halogens (X₂) to form metal halides (Reaction 2), all of which are ionic except for BeCl₂. Except for beryllium, the alkaline-earth metals react with water to produce very basic metal hydroxide solutions and hydrogen gas (Reaction 3). The most difficult alkaline-earth metal to study is radium, because it is a rare radioactive element that is produced from the nuclear decay of uranium.  The most common natural isotope, radium-226, has a half-life of 1,600 years and decays by alpha emission into radon-222 (Reaction 4). -When comparing atoms across the same row of the periodic table, which of the following groups will contain the atom with the lowest second ionization energy?

The radioisotope gallium-68 is used in nuclear medicine as a traceable radioactive emitter for diagnostic imaging procedures.  The ⁶⁸Ga isotope is generated from ⁶⁸Ge by:

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. -If a student adds bromothymol blue as an indicator to determine the amount of titratable acid in 25 mL of urine according to Folin's method, what effect will the bromothymol blue have on the pH of the urine sample?

The mean serum urate concentration (from uric acid) in a sample of urine was measured to be 64 mg/L.  If a 2.0 mL aliquot of the urine sample is analyzed, what is the total mass of the serum urate present in the aliquot?

Passage The radiopharmaceutical 2-deoxy-2-(¹⁸F)fluoro-D-glucose (abbreviated as ¹⁸F-FDG) is a modified form of D-glucose in which the hydroxyl group on the 2-carbon of the molecule has been replaced with a positron-emitting ¹⁸F radionuclide.  Synthesis of ¹⁸F-FDG is accomplished in several steps involving both nuclear and chemical reactions, as shown in Figure 1. Figure 1  Synthetic scheme for 2-deoxy-2-(¹⁸F)fluoro-D-glucoseInitially, a target of ¹⁸O-enriched water (¹⁸OH₂) is bombarded by protons using a cyclotron to achieve a (p-n) nuclear reaction in which a proton hitting the ¹⁸O nucleus is absorbed by the nucleus, causing the ejection of a neutron and forming an ¹⁸F nucleus.  Following separation of the resulting ¹⁸F⁻ anions from the residual ¹⁸OH₂ via an ion exchange column, ¹⁸F⁻ is used in a nucleophilic substitution reaction with an acetylated mannose triflate substrate.  Subsequent hydrolysis to remove the acetyl protecting groups yields ¹⁸F-FDG.Because ¹⁸F-FDG is an analog of D-glucose, it is readily taken into cells by the same metabolic processes as D-glucose.  Once inside the cells, the positron emissions of the ¹⁸F radionuclide can be detected by positron emission tomography (PET) scans to produce diagnostic medical images of objects such as tumors.  Emissions from the ¹⁸F radionuclide subsequently diminish over time via radioactive decay (Figure 2). Figure 2  Nuclear decay of the ¹⁸F radionuclideBecause of limitations in the spatial resolution of PET scanners, when an object (such as a tumor) is smaller than 30 mm in diameter, the measured specific uptake value (SUV) of ¹⁸F-FDG accumulated within the object is lower than the actual SUV.  The recovery coefficient (the ratio of the measured SUV to the actual SUV) shows that the measured SUV decreases as the size of the object decreases below the 30-mm threshold (Figure 3). Figure 3  Recovery coefficient as a function of object size in a PET scan with a spatial resolution of 7 mm using ordered subsets expectation maximization (OSEM) image processing Adapted from: S. Yu. "Review of F-FDG Synthesis and Quality Control." Biomed Imaging Interv J. ©2006 Department of Biomedical Imaging, Faculty of Medicine, University of Malaysa; and P. E. Kinahan, et al., "Positron emission tomography-computed tomography standardized uptake values in clinical practice and assessing response to therapy." Semin Ultrasound CT MR. ©2010 Elsevier. -Based on the imaging resolution data provided in the passage, what measured specific uptake value (SUV) is likely to be observed when imaging a 1.7-cm tumor with an actual SUV of 3.0?

Passage The alkali metals are a highly reactive family of metals occupying Group 1 (1A) of the periodic table.  In their elemental form, the alkali metals have a shiny, metallic appearance, but they are also very soft and can be easily cut by hand.  Due to their high reactivity, however, the alkali metals do not exist in their elemental form in nature.Because the alkali metals possess only a single valence electron with a low first ionization energy, the alkali metals (M) readily react with nonmetals and lose this electron to form cations with a +1 charge (M⁺) that are incorporated into ionic compounds.  Consequently, elemental samples of alkali metals will rapidly tarnish in oxygen by forming oxides (Reaction 1) and vigorously react with elemental halogens (X₂) to form halide salts (Reaction 2).2 M(s) + O₂(g) → M₂O(s)Reaction 12 M(s) + X₂(g) → 2 MX(s)Reaction 2Alkali metals also react violently with water to form very basic metal hydroxide (alkali) solutions while releasing hydrogen gas in the process (Reaction 3).2 M(s) + 2 H₂O(l) → 2 MOH(aq) + H₂(g)Reaction 3As demonstrated by these reactions, alkali metals possess a low electron affinity that makes accepting an additional electron to form an anion less favorable than losing the valence electron to form a cation.Of the alkali metals, francium is the rarest since it is a trace radioactive byproduct formed from the decay of uranium ores and has a very short half-life.  As a result, any francium formed will quickly decay into radium or astatine by beta decay or by alpha decay, respectively.Whereas hydrogen is a part of Group 1 based on its number of valence electrons, hydrogen is not an alkali metal.  Interestingly, some research has suggested that under extreme pressures and very low temperatures, hydrogen may form a solid with metallic properties. -At higher concentrations, alkali metal ions close to the size of a potassium ion can displace potassium ions in biological systems and promote low potassium levels (hypokalemia).  Which cation is most likely to displace K⁺ ions?

Litmus paper will turn blue when immersed in all of the following EXCEPT:

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. -Compared to ΔG° for the dissociation of acetic acid at a temperature yielding a pH of 2.6, ΔG° for the dissociation of acetic acid at another temperature yielding a pH of 2.0 would be:

Which of the following pressure measurements is equivalent to a pressure of 152 mmHg?0.2 atm20.3 kPa76 torr

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. -Assume that the pressure in a 1500 L monoplace chamber is increased from 1 ATA to 2 ATA.  Which expression gives the number of moles of oxygen that is added to the chamber if the temperature remains constant at 27 °C?

The density of ethanol varies slightly with temperature, as shown in the table below. Based on this data, how many molecules of ethanol are contained in 30 mL of ethanol at 8 °C?  (Note: The molar mass of ethanol is 46.08 g/mol.)

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. -Which of the following is the base dissociation constant K_b for the conjugate base of HCN?

Passage The alkaline-earth metals are a highly reactive family of metals occupying Group 2 (IIA) of the periodic table.  In their elemental form, the alkaline-earth metals have a shiny, silvery-white to slightly yellowish appearance.  The alkaline-earth metals exhibit some similarities to the chemistry of the alkali metals (Group 1) but are a little less reactive.Atoms of the alkaline-earth metals (M) have only two valence electrons and a low first and second ionization energy.  Consequently, these metals readily lose both electrons to produce cations with a +2 charge (M⁺²) during reactions with nonmetals.  As a result, the alkaline-earth metals do not exist in their elemental metallic form in nature.  Instead, the Group 2 elements are found primarily as metal oxides, which form by reacting oxygen (Reaction 1). Similarly, elemental samples of alkaline-earth metals will react with elemental halogens (X₂) to form metal halides (Reaction 2), all of which are ionic except for BeCl₂. Except for beryllium, the alkaline-earth metals react with water to produce very basic metal hydroxide solutions and hydrogen gas (Reaction 3). The most difficult alkaline-earth metal to study is radium, because it is a rare radioactive element that is produced from the nuclear decay of uranium.  The most common natural isotope, radium-226, has a half-life of 1,600 years and decays by alpha emission into radon-222 (Reaction 4). -Which of the following is the electron configuration of a Ca²⁺ ion?

Passage The alkaline-earth metals are a highly reactive family of metals occupying Group 2 (IIA) of the periodic table.  In their elemental form, the alkaline-earth metals have a shiny, silvery-white to slightly yellowish appearance.  The alkaline-earth metals exhibit some similarities to the chemistry of the alkali metals (Group 1) but are a little less reactive.Atoms of the alkaline-earth metals (M) have only two valence electrons and a low first and second ionization energy.  Consequently, these metals readily lose both electrons to produce cations with a +2 charge (M⁺²) during reactions with nonmetals.  As a result, the alkaline-earth metals do not exist in their elemental metallic form in nature.  Instead, the Group 2 elements are found primarily as metal oxides, which form by reacting oxygen (Reaction 1). Similarly, elemental samples of alkaline-earth metals will react with elemental halogens (X₂) to form metal halides (Reaction 2), all of which are ionic except for BeCl₂. Except for beryllium, the alkaline-earth metals react with water to produce very basic metal hydroxide solutions and hydrogen gas (Reaction 3). The most difficult alkaline-earth metal to study is radium, because it is a rare radioactive element that is produced from the nuclear decay of uranium.  The most common natural isotope, radium-226, has a half-life of 1,600 years and decays by alpha emission into radon-222 (Reaction 4). -The rate-determining step for the overall reaction in Figure 1 is the conversion of:

Which of the following values gives the number of parts per million (ppm) of NaCl in a 0.90% saline solution?

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 -When 0.1 M LaCl₃ is added to a sample, the following reactions take place: Ca3PO42s⇄ 3 Ca2++2 PO43- 2 La3++2 PO43-→2 LaPO4sIf 2 × 10⁻⁴ moles of Ca₃(PO₄)₂ react, the maximum number of grams of LaPO₄ (MW = 234 g/mol) that can be produced is:

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. -The experiment was originally performed at physiological pH (7.4).  If the pH were raised to 12, how would the corrosion rate of the magnesium metal change?

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 -Assume that a NiCd battery charges for 30 minutes (t) with an applied current (I) of 5.0 A.  Which expression could be used to calculate the number of moles of Cd produced on the cathode from electrolytic plating?

Which of the following element groups is NOT included in the representative elements?

Researchers wished to mimic the conditions of the medial Golgi (pH ≈ 6.3).  Which of the buffers shown in Table 1 would be best suited for this experiment?Table 1  Acid Dissociation Constants for Several Buffers