WYSE Academic Challenge State Finals Chemistry 2005 SOLUTION SET 1. Correct answer: d. Magnesium nitrate has the formula Mg(NO 3 ) 2, so 4.0 mol of magnesium nitrate dissolves to form 8.0 mol of nitrate ions. 2. Correct answer: b. Multiplying the mass of one atom by Avogadro s number, we get 28.0 g/mol. This is the molar mass of carbon monoxide (CO). 3. Correct answer: c. For a sample to have twice the average kinetic energy, the temperature as measured in Kelvin must be doubled. A temperature of 25 C is equal to 298K. Twice this is 596K, which is equal to 323 C. 4. Correct answer: b. Since we are given equal masses of each, the compound with the greatest percent by mass of phosphorus is the one that contains the greatest mass of phosphorus. Magnesium phosphide has the formula Mg 3 P 2. This compound is 45.9% phosphorus by mass. 5. Correct answer: c. Assume 100.0 g of the compound. 46.7 g nitrogen = 3.33 mol N; 53.5 g oxygen = 3.34 mol O. Thus we have a 1:1 mole ratio of N:O. 6. Correct answer: a. A 100.0 ml sample of 0.800 M NaOH contains 0.0800 mol NaOH, and a 600.0 ml sample of 0.100 M NaOH contains 0.0600 mol NaOH. Adding these, we get 0.0140 mol NaOH in 700.0 ml, which is 0.200 M NaOH. 7. Correct answer: a. A 245 ml sample of 1.20 M sugar has 0.294 mol sugar. For the concentration to be 1.30 M (since all of the sugar stays in solution and does not evaporate with the water), there must be 226 ml left (0.294 mol/0.226 L = 1.30 M). So, 245-226 = 19 ml must have evaporated. 8. Correct answer: b. 10.0 g of Zn is 0.153 mol Zn, which can produce 0.153 mol hydrogen; 10.0 g Ca is 0.250 mol Ca; 10.0 g Mg is 0.411 mol Mg. Thus, the least hydrogen is produced from the Zn, which gives rise to the smallest balloon. The HCl is limiting with Mg, but there are still 0.250 mol hydrogen produced.
9. Correct answer: e. Lead(II) nitrate is Pb(NO 3 ) 2 and sodium iodide is NaI. So, we have 16 mol nitrate ion and it does not form with the precipitate. We also have 8 mol lead(ii) ion and 15 mol iodide ion. The formula for lead(ii) iodide is PbI 2, so the iodide ion is limiting. 10. Correct answer: d. The molar mass of A is greater, so we have fewer moles of A than B (the number of moles equal the mass of the sample divided by the molar mass). If more moles of A are required according to the balanced equation (as it states in choice d), then A must be limiting. 11. Correct answer: d. In an isoelectronic series, all species have the same number of electrons (same electron configuration). So, most negatively charged species will have the least number of protons, so it will have the largest radius; the most positively charged species has the greatest number of protons, so it will have the smallest radius. 12. Correct answer: c. There are 20 valence electrons (7 from each atom, with one taken away to make it a positive charged ion). The species has a tetrahedral geometry and bent shape (similar to water). 13. Correct answer: d. The non-metal ion has 10 electrons, and a charge of 1- (since two of these ions balance the Ca 2+ ion). Thus, the neutral non-metal has 9 electrons, which makes it F. 14. Correct answer: c. Since the process is endothermic, the addition of heat pushes the reaction to the right (product side). This makes more H 3 O + (or H + ), and since ph = -log[h + ], the ph must decrease. However, more OH - is made as well (equal to the H + ), so the water remains neutral. 15. Correct answer: e. CN - acts as a base in water, and the Ka value for NH 4 + is greater than the Kb value for F -. 16. Correct answer: a. Acetic acid (HC 2 H 3 O 2 ) is a much stronger acid than HCN. In fact the amount of H + produced by HCN is so small compared to that from acetic acid, we can consider it to be negligible. The ph of 1.0 M acetic acid is 2.37. 17. Correct answer: d. For a solution to be buffered it must contain a weak acid and its conjugate base. In choice d, both NH 4 + (made in the reaction) and NH 3 (in excess) are present.
18. Correct answer: c. The value of the equilibrium constant for AgCl in Br - is given as Ksp(AgCl)/Ksp(AgBr) = 320. Using this to solve for the solubility of AgCl in Br -, we get 5.0 M. Use the Ksp for AgCl to solve for the solubility of AgCl in water to get 1.26 x 10-5 M. 19. Correct answer: b. This is one of the signposts that a system has reached equilibrium. 20. Correct answer: a. Use ln(a) = -kt + ln(a 0 ), in which A = 58, t = 17 min, and A 0 = 100. Students who use A = 42 will get choice b. 21. Correct answer: a. If the fluoride ions are in a simple cubic lattice, there is 1 net ion per cube. If the metal ions are in every other cube, the ion must have a charge of 2+, so choice a (Ca 2+ ) is the best answer. 22. Correct answer: b. Free energy ( G) changes determine spontaneity. 23. Correct answer: d. If the ionic compound did not contain the hydroxide ion (but had the same formula), the answer would be a. However, pure water has [OH - ] = 1.0 x 10-7 M (at 25 C). So we can assume that iron(iii) hydroxide dissolving in water adds a negligible amount of the hydroxide ion to the solution. 24. Correct answer: a. Let A = mole fraction of liquid A in solution and B = mole fraction of liquid B in solution (such that A + B = 1). The mole fraction of A above the solution can be solved for as But this is twice that of the mole fraction of A in solution, so A(350) A(350) + B(100) A(350) A(350) + B(100) = 2A Solving for A, we get 0.300.
25. Correct answer: e. We cannot tell anything about the kinetics of the reaction (so a is not necessarily true). The reaction will occur to some extent, although the value of K will be small (so b and d are not true). The reaction as written shows an increase in entropy. For G to be positive, the reaction must be endothermic, so c is not true. This leaves choice e. 26. Correct answer: b. The equation is 4H + + Cr + NO 3 - Cr 3+ + NO + 2H 2 O. 27. Correct answer: a. Many students assume b is true, although it is not necessary. Only a is necessary. 28. Correct answer: d. Choice a should be 2,3-dichloropentane. Choice b should be 3,3-diethyl-2- methylpentane; choice c does not need the cis. 29. Correct answer: b. Adding the same mass of neon as helium increases the number of moles of gas, but does not double it (since neon has a larger molar mass than helium). So the pressure increases, but does not double. 30. Correct answer: e. The standard potential for the cell is 0 V since the same species are in each side of the cell; the reduction potential cannot be determined with this information. 31. Correct answer: e. The species all have the same electron configurations. However, magnesium has the greatest number of protons, so the radius is the smallest. 32. Correct answer: b. The electron to remove from the Ca + ion comes from a higher energy level than the others, so it is easier to remove. 33. Correct answer: e. All of the molecules are bent. 34. Correct answer: b. The gases may be different so that each container would have a different mass of gas, but for P, V, and T to be the same, n must be the same.
35. Correct answer: a. If we assume 1.0 L (and use PV=nRT), we get n = 0.0409 mol. For neon, 0.0409 mol = 0.825 g; so the density is 0.825g/L. 36. Correct answer: e. Hydrogen bonds give rise to attraction between molecules. 37. Correct answer: c. 38. Correct answer: a. The smaller the wavelength, the greater the frequency. 39. Correct answer: e. If we use the law of mass action, we get Kp = 1899.335. To convert Kp to K in this case we must multiply by RT, getting e. 40. Correct answer: b. The temperature of water remains constant at 100 C.