WYSE Academic Challenge Sectional Chemistry 2005 SOLUTION SET 1. Correct answer: c. Since the ion has a 2- charge, this means there are two extra electrons as compared to protons, so the ion must be sulfur (which has 16 protons). 2. Correct answer: c. Compounds must consist of two or more different elements (atoms). 3. Correct answer: b. 100.0 g of CO 2 contain 4.544 moles of oxygen ([100.0 / 44.01] x 2). 100.0 g of H 2 O contain 5.551 moles of oxygen ([100.0 / 18.02] x 1). 1.00 mole of CO 2 contains 2.00 moles of oxygen. 2.00 moles of H 2 O contain 2.00 moles of oxygen. Since 100.0 g of H 2 O contain the greatest number of moles of oxygen, it also contains the greatest number of oxygen atoms (since 1 mole = 6.022 x 10 23 oxygen atoms). 4. Correct answer: d. Since real gas particles have attraction for one another, real gases act more ideally at higher temperatures because they are moving faster and spend less time closer to each other (and thus are more elastic). 5. Correct answer: c. There are 0.0200 mol Cl - ions from the NaCl since 0.1000 L x 0.200 M = 0.0200 mol NaCl, and there is a 1:1 ratio between NaCl and Cl - ions when NaCl is in an aqueous solution. There are 0.0750 mol Cl - ions from the CaCl 2 since 0.2500 L x 0.150 M = 0.0375 mol CaCl 2, and there is a 1:2 ratio between CaCl 2 and Cl - ions when CaCl 2 is in an aqueous solution. To obtain the total concentration of Cl - ions, divide the total moles of Cl - ions with the total volume of solution ([0.0200 + 0.0750moles] / [0.1000 + 0.2500 L] = 0.271 M Cl -. 6. Correct answer: a. The balanced equation is 2H 2 + O 2 2H 2 O. Since we have equal masses, assume we re starting with 10.0 of each reactant. If we have 10.0 g of H 2, then 79.4 g O 2 is needed since (10.0gH 2 )(1molH 2 /2.016gH 2 )(1molO 2 /2molH 2 )(32.00gO 2 /1molO 2 ). Since we only have 10.0g of oxygen, this makes oxygen gas the limiting reactant. 7. Correct answer: a. One mole of Na 3 PO 4 produces 4 mol of ions (3Na + + PO 4 3- ). One mole of KCl produces 2 mol of ions (K + + Cl - ). One mole of Fe(NO 3 ) 2 produces 3 mol of ions (Fe 3+ + 2NO 3 - ). One mole of NaOH produces 2 mol of ions (Na + + OH - ).
8. Correct answer: d. An ionic solid consists of at least one cation and one anion and it could contain more. Therefore, when 1 mole of these ionic solids are dissolved in water, they will produce at least two moles of ions. Some examples are NaCl Na + + Cl - and Na 2 CO 3 2Na + + CO 3 2-. 9. Correct answer: c. First, determine the [OH - ] when the ph = 12.00 (poh = 14.00 12.00 = 2.00 and poh = -log[oh - ], so [OH - ] = 0.010 M). Since you have 100.0 ml of solution, 0.0010 mol of [OH - ] is present (0.010M x 0.1000L). Next, determine the [OH - ] when the ph = 11.00 (poh = 14.00 11.00 = 3.00, so [OH - ] = 0.0010 M). Therefore, 1.0 L of aqueous NaOH solution is required for the new ph (0.001mol/0.001M). So, the volume of water that must be added is 1000 ml 100 ml = 900 ml. 10. Correct answer: b. Even though NaOH is a strong base, the concentration of [OH - ] is so small that the autoionization of water has a stronger effect on the ph (since [OH - ] for water is 1.0 x 10-7 M). 11. Correct answer: c. The correct name is magnesium chloride. 12. Correct answer: d. The equilibrium expression is determined by multiplying the product concentrations and dividing this by the multiplication of the reactant concentrations. But, we assume the concentration of liquid water remains constant so this is not included in the expression. Also, the coefficients in the reaction become exponents in the equilibrium expression. 13. Correct answer: a. The anion of the weak acid, which remains in solution at the stoichiometric point, is a base, therefore the ph is greater than 7.00 (making a true). Even in a strongly acidic solution, some [OH - ] is present but only in tiny amounts (making b false). Because ph is based off of a logarithmic scale, there is not a direct relationship between ph and the concentration of [H + ]. For example, when ph = 2.00, [H + ] = 0.010 M (ph = -log[h + ]). When ph = 1.00, [H + ] = 0.10 M. Therefore, the smaller the ph, the higher the [H + ] (and in this case, the concentration is 10 times greater as the ph lowers). This makes c false. 14. Correct answer: e. Use the Henderson-Hasselbach equation (ph = pka + log[f - ]/[HF]) to determine that [HF] needs to be less than [F - ]. The pka of HF is 3.14 (pka = -log{ka}). Therefore, in order for the ph to be 4.00, the log[f - ]/[HF] must be a positive value.
15. Correct answer: c. Chemistry For CF 4, PF 5, and BF 3, the dipole moments cancel and make these molecules nonpolar. Only SF 4, with a seesaw shape, is polar (there is a lone pair on the sulfur). 16. Correct answer: a. When in standard form, a balanced equation can have a coefficient of 1 or greater (but cannot be fractions). 17. Correct answer: b. Since the two containers are at the same volume and temperature, the pressure and moles of gas are directly related to each other (using PV = nrt). Assume there are 10.0g of helium in one tank and 10.0g of argon in the other tank. Converting these to moles we get 2.50 mol He (10.0/4) and 0.250 mol Ar (10.0/40). Since the moles of He are 10 times greater than the moles of Ar, the pressure is also 10 times greater. 18. Correct answer: d. Assume there are 43.6g of phosphorus which results in 1.41 mol P (43.6/30.97). Assume there are 56.4g of oxygen (100.0-43.6) which results in 3.53 mol O (56.4/16.0). Divide each by the smallest number of moles to get the smallest whole number ratio. For phosphorus, the ratio is 1.41/1.41 = 1. For oxygen, the ratio is 3.53/1.41 = 2.50. Empirical Formulas must not have fractions so multiply each number by 2 to get P 2 O 5. Since the molar mass is twice that of the empirical formula, the molecular formula must be P 4 O 10. 19. Correct answer: a. To determine the identity of the other gas, calculate the molar mass (#gram/#moles). Since the two containers are at the same volume and temperature, the pressure and moles of gas are directly related to each other (using PV = nrt). 25.0g of Ar = 0.626 mol Ar (25.0/39.9). The pressure of the unknown gas is twice that of argon, so the # of moles of unknown gas is 1.25 (0.626 x 2). Therefore, the molar mass is 16.0 g/mol (20.0/1.25), which is CH 4. 20. Correct answer: e. 21. Correct answer: e. All of the electron configurations are correct. 22. Correct answer: e. C 2 H 4, CO 3 2-, and O 3 contain double bonds. HCN contains a triple bond. 23. Correct answer: b. 24. Correct answer: a. CH 4, with a tetrahedral shape, is nonpolar because the dipoles cancel. London dispersion forces are the strongest intermolecular forces for methane. NH 3, with a trigonal pyramid shape, is polar. Ammonia exhibits hydrogen bonding which is stronger than London dispersion forces.
25. Correct answer: e. The % mass of Fe in the unknown iron oxide compound is 69.9% (5.00/7.15 x 100). Assume there are 69.9g of Fe which results in 1.25 mol Fe (69.9/55.85). Assume there are 30.1g of oxygen (100.0-69.9) which results in 1.88 mol O (30.1/16.0). Divide each by the smallest number of moles to get the smallest whole number ratio. For iron, the ratio is 1.25/1.25 = 1. For oxygen, the ratio is 1.88/1.25 = 1.50. Empirical Formulas must not have fractions so multiply each number by 2 to get Fe 2 O 3. 26. Correct answer: e. The balanced equation: 3Ca(NO 3 ) 2 + 2Na 3 PO 4 6NaNO 3 + Ca 3 (PO 4 ) 2 The sum of the coefficients: 3+2+6+1=12 27. Correct answer: a. To determine the identity of the gas, calculate the molar mass (#gram/#moles). Use the ideal gas law (PV=nRT) to find the moles of gas. Moles of gas = 0.115 (n = PV/RT). Make sure the temperature is converted to Kelvin (45+273). The molar mass is 38.0 g/mol (4.37/0.115), which is F 2. 28. Correct answer: a. Removing electrons from an atom always requires energy. 29. Correct answer: b. For unit cells, anions and cations are represented by spheres. Since there is an anion at each corner of the cube, this comprises one complete anion (? per corner x 8 corners = 1 anion). In addition, there is an anion in the center of the cube so there are a total of 2 anions in the unit cell. There are cations at the center of each face of the cube, which means that there is only half a sphere at each face. Thus, there are a total of 3 cations in the unit cell (½ at each face x 6 faces = 3 cations). The compound must be Ca 3 (PO 4 ) 2. 30. Correct answer: d. The general equation is Rate = k[a] n [B] m. Using the data given, solve for m by dividing the rate data from row #3 by the rate data from row #2: {(9.6x10-2 ) / (2.4x10-2 )} = {[0.80] m / [0.40] m }, so m = 2. Next, solve for n by dividing the rate data from row #2 by the rate data from row #1 (and substitute your value for m): {(2.4x10-2 ) / (2.0x10-3 )} = 4{[0.30] n / [0.10] n }, so n = 1. The equation is Rate = k[a][b] 2. 31. Correct answer: e. CCl 4 is a liquid at room conditions because the London dispersion forces are greater than in CH 4 (because CCl 4 has more electrons than CH 4 ). They both have the same geometries and shapes and are both nonpolar. 32. Correct answer: d. The balanced equation is 2Na(s) + 2H 2 O(l) 2NaOH(aq) + H 2 (g).?h rxn = S?H prod S?H react.?h rxn = [2(-470)] [2(-286)] = -368 kj.
33. Correct answer: b.?s (change in entropy of the system) is negative for the dissociation of HF because of the high degree of ordering that occurs as the water molecules associate with the small F - ion. Although the enthalpy of hydration favors dissociation of HF, the entropy of hydration strongly opposes it. 34. Correct answer: d. Molar mass is determined by #grams/#moles. The mass of the solute is 1.00 g but the # of moles still has to be calculated. This can be found by multiplying the molality by the kg of solvent (since molality = moles of solute / kg of solvent). Use?T = K f m solute to determine the molality first. The molality = 0.318 mol/kg (m solute =?T / K f, m solute = 0.592 / 1.86). The moles of solute = 0.00318 mol (0.318mol/kg x 0.0100kg). Therefore, the molar mass is 314 g/mol (1.00 / 0.00318). 35. Correct answer: e. A spontaneous reaction requires a positive e cell voltage. The half reaction for silver is Ag Ag + + e - (silver is the reducing agent so the reaction is an oxidation reaction in which e o = -0.80V). Silver is not able to spontaneously reduce any of the metals listed because e cell would be negative. 36. Correct answer: b. One tetramethylbenzene structure has a methyl group on positions 1,2,3,4. Another tetramethylbenzene structure has a methyl group on positions 1,3,4,5. The final tetramethylbenzene structure has a methyl group on positions 1,2,4,5. 37. Correct answer: d. Choose the longest chain (which is hexane) and assign the methyl groups to the lowest numbers (2,3,4). 38. Correct answer: c. The first half reaction is: 5e - + 8H + - + MnO 4 Mn 2+ + 4H 2 O. The second half reaction is: 2- H 2 O + SO 3 SO 2-4 + 2H + + 2e -. Therefore, the common factor for both half reactions (to balance the number of electrons) is 10. 39. Correct answer: c. Since the temperature of the mixture increases, heat must have been released so the reaction is exothermic. Thus, the enthalpy change is negative (since the reaction is at constant pressure). 40. Correct answer: b. A positron is a particle with the same mass as the electron but opposite charge.