04/08 Chem
What makes a buffer solution?
A buffered solution consists of a weak acid and a salt of its conjugate base or a weak base and its conjugate acid. Buffers resist changes in pH by neutralizing added H+ and OH− ions. The pH of a buffered solution depends on the ratio of weak acid and its conjugate base
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: A.higher.[9%] B.the same.[16%] C.2 times lower.[35%] D.4 times lower.[38%]
Accordingly, the equation for the dissolution of NaCl shows a 1:1 mole ratio of Cl− to NaCl: NaCl→Na++Cl− If a solution volume of 1 L is assumed as a convenient basis for comparison, then 5 mM of NaCl contains 5 millimoles of Cl−. In contrast, the equation for the dissolution of MgCl2 shows a 2:1 mole ratio of Cl− to MgCl2: MgCl2→Mg2++2Cl− Consequently, 1 L of a 10 mM solution of MgCl2 contains 20 millimoles of Cl−. Dividing the number of millimoles of Cl− in the NaCl solution by the number of millimoles of Cl− in the MgCl2 solution gives: 5 mmol20 mmol=5⋅14⋅5=14 Therefore, the number of moles of Cl− in the 5 mM NaCl solution is 4 times lower than the number of moles of Cl− in an equal volume of the 10 mM MgCl2 solution.
Which of the following experimental changes will decrease the percent recovery of oxytocin in Experiment 1? A.Adding more sodium citrate[26%] B.Decreasing the initial concentration of oxytocin[43%] C.Cooling the samples after 4 weeks[15%] D.Increasing the buffer concentration[13%]
Accordingly, the pH of a buffered solution depends on the concentration ratio of acid to conjugate base. Adding more acid will decrease the pH and adding more base will increase the pH. In Experiment 1, the citrate buffer is made of citric acid [HA] and sodium citrate [A−]. Adding more sodium citrate will increase the pH of the solution. The passage states that oxytocin is more stable at a pH of 4.5, and Figure 1 shows that at the higher pH of 6.5 (water), less oxytocin was recovered.
The Nernst equation states: E=E°−0.0592nlog([X]anode[Y]cathode) For a concentration cell that has a potential of 0.089 V and utilizes Ni2+ solutions at the anode and the cathode, which of the following statements correctly describes the molar cell concentrations [Ni2+]anode and [Ni2+]cathode and the direction of electron flow within the cell? A.[Ni2+] is greater at the anode; electrons flow from cathode to anode[8%] B.[Ni2+] is greater at the cathode; electrons flow from anode to cathode[45%] C.[Ni2+] is greater at the anode; electrons flow from anode to cathode[36%] D.[Ni2+] is greater at the cathode; electrons flow from cathode to anode[9%]
Because the measured potential for the cell is positive (E = 0.089 V), the ion concentrations must have a ratio of [X]anode/[Y]cathode that is less than one so that the logarithmic term of the Nernst equation will be negative. A negative logarithmic value multiplied by the negative constant in the equation results in a positive value for E. Therefore, [Ni2+] is greater at the cathode, and the electrons in this cell will flow from the anode to the cathode.
Concentration cell
Concentration cells use the same electrode material and ionic solution at both the anode and the cathode but operate by a concentration difference between the cells, which produces a difference in potential. Electrons will migrate from the anode (fewer cations) to the cathode (more cations) until the concentrations equalize
The chemical structures of sulfur compounds H2S, SF6, S2Cl2, and S4N4 each contain only single (sigma) bonds. The compound that contains the longest bond between sulfur and an atom of another element is: A.H2S[43%] B.SF6[16%] C.S2Cl2[27%] D.S4N4[13%]
Covalent bonds between atoms are formed by sharing electrons, which is made possible through the overlap of the atomic orbitals from each atom. Covalent bonds made by the end-to-end overlap of atomic orbitals are called sigma (σ) bonds, whereas covalent bonds made by the side-to-side overlap of p orbitals are called pi (π) bonds. A single covalent bond consists only of a σ bond, a double bond consists of one σ bond and one π bond, and a triple bond consists of one σ bond and two π bonds. Because a σ bond involves end-to-end overlap of two orbitals, the length of a σ bond can be estimated as the sum of the atomic radii of the bonded atoms. Bonding between atoms with larger atomic radii positions the atomic nuclei farther apart and results in a longer σ bond (and vice versa). On the periodic table, atomic radii tend to decrease across a row (due to an increasing effective nuclear charge) and increase down a column (due to having an additional electron shell). In this question, the given compounds all have σ bonds to a sulfur atom. As a result, the longest bond between sulfur and another element will be formed with the element that has the largest atomic radius. Because Cl has a larger atomic radius than H, N, or F, the S-Cl bond in S2Cl2 will the longest bond with sulfur among the given compounds.
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? A.Na+[19%] B.F−[27%] C.Mg2+-correct D.O2−[13%]
Ionic radii tend to decrease in size across a period (row) of the periodic table (left to right) and increase moving down a group (column). This trend occurs for metal cations, and then resets and repeats for anions beginning near the division between metals and nonmetals, past which anions tend to preferentially form. Compared to the neutral atom of a given element, its cation will be smaller but its anion will be larger. Losing electrons to form a cation causes the remaining electrons to experience a greater effective nuclear charge (Zeff), pulling the electrons closer to the nucleus. Conversely, gaining electrons to form an anion produces greater electronic repulsion and nuclear shielding (lesser Zeff), which pushes electrons farther from the nucleus. Na+, F−, Mg2+, and O2− ions are isoelectronic (have the same number of electrons), but because the number of protons is different in each ion, the electrons in each ion experience a different Zeff. Therefore, in an isoelectronic series, ionic radii decrease with increasing atomic number. Because magnesium has the highest atomic number (greatest number of protons) in the isoelectronic series, Zeff is greatest in this ion, making it the smallest within the given series.
Isotopes
Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons in the nucleus. Isotopes of the same element have nearly identical chemical properties but differ in their physical properties.
Consider the dimerization of nitrogen dioxide at room temperature: 2 NO2(g)⇌ N2O4(g) Keq = 8 atm-1 If the equilibrium partial pressure of nitrogen dioxide in a container is 0.5 atm, what is the partial pressure of dinitrogen tetroxide? A.0.03 atm[10%] B.0.5 atm[17%] C.2 atm[48%] D.4 atm[23%]
Nitrogen dioxide (NO2) can combine with itself to form dinitrogen tetroxide (N2O4). The equilibrium constant for this reaction (Kp = 8 atm−1) is the ratio of N2O4 to NO2 when equilibrium is achieved. However, because two NO2 molecules are required for each reaction, the NO2 partial pressure must be squared. Therefore, the equilibrium partial pressures of NO2 and N2O4 in the described scenario are related by the equation 8 atm−1= PN2O4 atm0.52 atm2 This equation can be rearranged and solved as follows: PN2O4 atm = (0.52 atm2) × 8 atm−1 = 0.25 atm2 × 8 atm−1 = 2 atm
Radioactive emission
Radioactive beta decay can occur in three forms: β−-decay (electron emission), β+-decay (positron emission), and electron capture. In β−-decay, a neutron converts to a nuclear proton and emits an electron. In β+-decay, a nuclear proton converts to a neutron (the opposite of β−-decay) and ejects a positron (an electron with a positive charge). In electron capture, a proton captures an electron near the nucleus and converts to a neutron without a positron or electron emission. In all three forms of beta decay, the mass number remains unchanged whereas the atomic number either increases (β−-decay) or decreases (β+-decay and electron capture) by 1. As the atomic number changes, the identity of the element changes accordingly.
What approximate volume of the oxytocin solution with the 10 mM Zn2+ additive was analyzed if 2.2 × 10−6 moles of oxytocin acetate (MW = 1067 g/mol) were recovered from the sample after 4 weeks at 50 °C? A.9 μL[14%] B.900 μL[23%] C.2 mL[32%] D.10 mL[29%]
Solution concentration can sometimes be expressed in terms of mass per unit volume. When concentration is stated in this way, the solution concentration can be used as a conversion factor to find the volume of solution that contains a given amount of mass. If the number of moles, instead of the mass, of a substance is known, the moles can be converted into the corresponding amount of mass using the molar mass. In Experiment 2, only 80% of the initial amount of oxytocin monoacetate was recovered from the sample with 10 mM Zn2+ additive after being stored for 4 weeks at 50 °C (Figure 2). Accordingly, if 2.2 × 10−6 moles of oxytocin acetate are recovered, this must be 80% of the original amount n, according to the equation: 0.8 n=2.2×10−6 mol Solving for n gives: n=2.2×10−60.8=2.75×10−6 mol Converting the moles to the corresponding mass using the molar mass and expressing the result in milligrams to match the units of concentration give: 2.75×10−6 mol×1067 g1 mol×1000 mg1 g=2.9 mg oxytocin acetate Finally, using the given solution concentration as a conversion factor, the original volume of analyzed solution is: 2.9 mg oxytocin acetate×1 mL solution0.3 mg oxytocin acetate=9.7 mL≈10 mL solution
Buffer range
The buffering range typically lies within one pH unit of the pKa (pH range = pKa ± 1).
Formal charge and oxidation state are two different ways of accounting for electrons between atoms. Based on the Lewis structure of the ClO− anion, which of the following pairs of values corresponds to the formal charge of the chlorine atom and the oxidation state of the oxygen atom, respectively? A.+1, −1[8%] B.0, −1[40%] C.0, −2[31%] D.−1, −2[18%]
The formal charge (FC) accounting method assumes that the electrons in a covalent bond are shared equally, and FC is calculated as: FC=(Groupvalence)−(Nonbondingelectrons)−12(Bondingelectrons) The oxidation state (OS) accounting method assigns all electrons in a given bond to the most electronegative atom of the bond, and OS is calculated as: OS=(Groupvalence)−(Nonbondingelectrons)−(Bondingelectrons) Applying these formulas to the Lewis structure of CIO−, the formal charge of the chlorine atom and the oxidation state of the oxygen atom are calculated to be 0 and −2, respectively. FCCl=(7)Groupvalence−(6)Nonbondingelectrons−12(2)Bondingelectrons=0 OSO=(6)Groupvalence−(6)Nonbondingelectrons−(2)Bondingelectrons=−2
What are the predominant species in the phosphate buffer system and in the ammonia buffer system in an aqueous solution at a pH of 8? A.H2PO4− and NH4+[11%] B.H2PO4− and NH3[22%] C.HPO42− and NH4+[46%] D.HPO42− and NH3[19%]
This relationship shows that when the pH of a solution is equal to the pKa of a weak acid, there are equal amounts of the weak acid and its conjugate base in solution (if pH = pKa, then [HA] = [A−]). If the pH is less than the pKa, the equilibrium is shifted toward a higher concentration of the nondissociated acid, and [HA] > [A−]. Alternately, if the pH is greater than the pKa, the equilibrium is shifted toward a higher concentration of the deprotonated conjugate base, and [HA] < [A−]. Accordingly, consider the equilibrium: H2PO4−⇄HPO2−4+H+ At a pH of 8.0, HPO42− will be the dominant species in the phosphate buffer because the pKa of H2PO4− is 6.8, which is less than 8. Likewise, consider the equilibrium: NH4+⇄NH3+H+ In this case, the acidic NH4+ will be the dominant species in the ammonia buffer because the pKa of NH4+ is 9.1, which is greater than 8.
D- glucose
its C-5 carries OH which is to the right.
