C. For any redox reaction, the easiest way to determine the total number of moles of transferred electrons is to determine the change in oxidation states of a given element over the course of the reaction, then multiply by the total number of moles of the element required in the reaction. Each reaction will have two such substances (one that gains electrons and one that loses them), and either substance will work. Choice A shows 5 mol Sn changing from a +2 to a +4 oxidation state, and 2 mol Mn7+ changing to Mn2+, both 10 mol e- transfers (eliminate choice A). Choice B shows 2 mol Fe going from a +3 to a +2 oxidation state, and 1 mole of Ti0 going to Ti2+, both 2 mol e- transfers (eliminate choice B). Choice C shows 3 mol Pb0 going to Pb2+, and 2 mol Cr6+ going to Cr3+, both 6 mol e- transfers. Choice D shows 2 mol S change from a +4 to a +5 oxidation state, while 2 mol O changes from -1 to -2 oxidation states, both 2 mol e- transfers (eliminate choice D). B. If the cation of a salt is the conjugate acid of a weak base, it will make a solution acidic. Similarly, if the anion of a salt is the conjugate base of a weak acid, it will make a solution basic. Ammonium perchlorate, NH4ClO4, contains the conjugate base (ClO4-) of a strong acid, which will have no reaction with water, and a conjugate acid (NH4+) of the weak base, NH3, which should make the solution acidic. Roman numeral I is therefore true (eliminate choice C). Silver nitrate, AgNO3, also contains the conjugate base (NO3-) of a strong acid, and its cation is reactive with water to form an insoluble hydroxide. Since Ag+ pulls hydroxide out of the solution, it lowers the pH of the solution by producing excess hydrogen ions (Ag+ + H2O → AgOH + H+). Roman numeral II is therefore also true (eliminate choices A and D). The anion in sodium acetate (C2H3O2-) is the conjugate base of a weak acid and will therefore make the solution basic according to the following hydrolysis equation: C2H3O2- + H2O → HC2H3O2 + OH-. The Na+ ion is not reactive with water since it is from the strong base NaOH. D. Galvanic cells are spontaneous electrochemical cells, so the E°cell must be positive (eliminate choice C); ΔG° must be negative for a spontaneous process (eliminate choice A). To create a positive standard cell voltage, the gold half-reaction should be reversed and become an oxidation half-reaction. This will make the E°cell = +1.087 + (-0.930 V) = +0.157 V (eliminate choice A, choice D is correct). Note that to calculate ΔG°, apply the equation ΔG° = -nFE°cell. The total number of electrons transferred, n, will be 6 based on the balanced equation from the half reactions: (2 Au(s) + 8 Cl- + 3 Br2(aq) → 6 Br- + 2 [AuCl4]-). This makes ΔG° = -(6 mol e-)(96,500 C/mol e-)(+0.157 V) ≈ -(6)(100,000)(0.16) ≈ -96,000 J, or just less than -96 kJ via estimation. The conjugate bases of HSO4-, CH3OH, and H3O+ are, respectively:
Question 18 Answer Choices
A. SO4-, CH2OH-, H2O
B. SO42-, CH3O-, H2O
C. SO4-, CH3O-, OH-
D. SO42-, CH2OH-, OH-
The conjugate bases of HSO4-, CH3OH, and H3O+ are, respectively SO42-, CH3O-, H2O.
The conjugate base of a chemical species is simply that species after it has lost an H+. Therefore, the conjugate base of HSO4- is SO42- (eliminating "SO4-, CH2OH-, H2O" and "SO4-, CH3O-, OH-"), and the conjugate base of H3O+ is just H2O (eliminating "SO42-, CH2OH-, OH-"). The most acidic proton on CH3OH is the hydroxyl proton, so CH2OH is the incorrect conjugate