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Chemistry 111 Sect. 3 Exam #1 October 5, 2000 Name: _____________________ KEY _____________________ Student #: ______________________________________________ Please put your name and student number on all pages in case they become separated. The last page of the exam has some useful information. The exam lasts 60 minutes. There are a total of 100 points. MULTIPLE CHOICE QUESTIONS Place an X in the box corresponding to the correct answer. 1. (5 pts) The name of the compound Na2SO4 is: o o oX o o nitrogen sulfide nitrogen sulfite sodium sulfate sodium sulfite nickel sulfide 2. (5 pts) An ion with the symbol 138 2+ 56 Ba is correctly described by which of the following? Numbers of protons, neutrons, electrons: o 56, 138, 56 o oX o o 69, 69, 54 56, 82, 54 56, 82, 56 56, 82, 58 The top number is mass number A; the bottom is atomic # Z. Z is # of protons in the nucleus, A is the sum of # of protons plus # of neutrons. In the neutral (uncharged) atom, the # of electrons equals the # of protons. For this particular ion, Z=56, A=138, and there are two fewer electrons than protons. Thus, # prot = 56, # neutr = 138 - 56 = 82, and # electr = 56 - 2 = 54. 3. (5 pts) You measure the three sides of a cube as 4.021 cm, 11.6 cm and 12.5 mm. The volume of a cube is length × width × height. What is the correct number of significant figures that you should use to report the measured volume of the cube? o o oX o o 1 2 3 The # of SF retained for a product of two numbers is the smaller # of SF of the two numbers. The smallest # of SF for the three numbers here is 3, so that is the correct # of SF in the final answer, which is 5.83 cm3. 4 can’t tell Page 1 Chemistry 111 Sect. 3 Exam #1 October 5, 2000 Name: _____________________ KEY _____________________ Student #: ______________________________________________ 4. (5 pts) Succinic acid has an empirical formula of C2H3O2 and a molecular weight of 118 grams/mol. What is the molecular formula of succinic acid? o o oX o o C8H12O8 C2H3O2 C4H6O4 C6H12O6 The empirical (simplest) formula, if that is also the true molecular formula would have a molecular mass of 59 g/mol (2 carbon + 3 hydrogen + 2 oxygen = 2•12 g/mol + 3•1 g/mol + 2•16 g/mol). The true succinic acid molecular mass is twice that, so there must be twice as many copies of C, H and O in the correct formula. C3H6O3 5. (5 pts) A 1.000 g sample of metal X combines with 0.2200 g of oxygen. Calculate the atomic mass of X in amu if the formula of the compound formed is XO2. o o o o oX 0.78 9.1 72.7 78 145.5 Start by determining the # of moles of oxygen that is combined with the metal: # mol of O = (0.2200 g)/(16 g/mol) = 0.01375 mol. Note that there are half as many moles of metal X as there are of oxygen in the compound, so # mol of X = (0.01375 mol)/2 = 0.006875 mol. Since 1 g of X makes up this 0.006875 mol, the atomic mass of X is (1.000 g)/(0.006875 mol) = 145.5 g/mol. The definition of amu, which is based on 12C, is that one 12C has a mass of exactly 12 amu. Molar mass is also based on 12C, which is defined to have a mass of exactly 12 g/mol. So the numerical value in g/mol is the same numerical value in amu for the elements. 6. (5 pts) When the following equation is balanced with the smallest whole number coefficients possible, the coefficient of KNO3 is: ___K3PO4 + ___Ca(NO3)2 → ___KNO3 + ___ Ca3(PO4)2 o o o o oX 2 3 4 5 It takes 3 Ca2+ for every 2 PO43- to make calcium phosphate. That means we need three calcium nitrates and two potassium phosphates, which leaves us with six potassium nitrates in the products. 6 7. (5 pts) What is the correct net ionic equation for the reaction (if a reaction occurs) between sulfuric acid and sodium hydroxide? Page 2 Chemistry 111 Sect. 3 Exam #1 October 5, 2000 o o o oX o Name: _____________________ KEY _____________________ Student #: ______________________________________________ H2SO4(aq) + 2 NaOH(aq) → Na2SO4(aq) + 2 H2O(l ) H2SO4(aq) + 2 OH-(aq) → SO42-(aq) + 2 H2O(l) Na+(aq) + SO42-(aq) → Na2(SO4)(aq) H+(aq) + OH-(aq) → H2O(l) No reaction Sulfuric acid and sodium hydroxide are strong electrolytes. They completely dissociate in water. The two ions that will react in the NIE are H+ and OH-. The other ions are soluble as Na2SO4 is a strong electrolyte. 8. (5 pts) The oxidation number of Mn in K2MnO4 is: o o o oX o First note that permanganate has a charge of -2. Now apply the rules of precedence for oxidation numbers. O is always -2 unless it is bound to F or another O, so the total charge associated with the 4 Os is -8. Since the permanganate carries a net -2 charge, then Mn in this polyatomic ion must have an oxidation # of (-2) (-8) = +6. -2 0 +2 +6 +7 MULTIPLE MULTIPLE CHOICE QUESTIONS Place an X in the boxes corresponding to the correct answers. Any number of answers may be correct, including none of them. 9. (10 pts) In lecture we discussed the reaction between the insoluble compound calcium hydroxide and sulfuric acid. In the demonstration, the calcium hydroxide dissolved when sulfuric acid was added to it. In this reaction, what are the spectator ions? Ca(OH)2(s) + H2SO4(aq) → CaSO4(aq) + H2O(l) o o o oX o Ca2+(aq) - OH (aq ) H+(aq) SO42+(aq ) CaSO4(aq) Calcium hydroxide is a weak base. Sulfuric acid is a strong acid. Thus, the species that are present as reactants are Ca(OH)2, H+ and SO42-. Because water is formed (this is an acid-base reaction), the two species that react are Ca(OH)2 and H+: Ca(OH)2 + H+ → Ca2+ + H2O Sulfate does not participate; it is the spectator. Ca2+ is not a spectator because it is only on the product side of the reaction. 10. (10 pts) You are asked to convert the number of grams of copper to numbers of atoms of copper. During the calculation, what units “cancel out”. Page 3 Chemistry 111 Sect. 3 Name: _____________________ KEY _____________________ Exam #1 Student #: ______________________________________________ October 5, 2000 To convert grams to atoms, we need to first convert grams to o mL moles and then use Avogadro’s number to convert moles to oX g number of atoms: o atoms [g]/[g/mol]•[# atoms/mol] oX mol o percent The units that have “canceled” are g and mol. 11. (10 pts) Which of the following are TRUE? o oX oX o o ¼ mole of N2 contains 3.0 × 1023 molecules 1 mole has 6×1023 molecules; ¼ has 1.5×1023 the density of hydrogen gas at room temperature and atmospheric pressure is less than that of air recall the balloons in the first class all redox reactions include reducing agents and oxidizing agents Always true the temperature at which water boils is close to 460 K 100 °C = (100 + 273) K = 373 K all ionic compounds dissolve readily in water precipitating salts don’t dissolve readily SHORT ANSWER QUESTIONS Answer in the spaces indicated. 12. (10 pts) When the following skeletal equation is balanced under acidic conditions, what are the coefficients of the species shown? 2+ _____ 1 Cd(s) + _____ 1 ClO4 (aq) → _____ 1 Cd (aq) + _____ 1 ClO3 (aq ) Ox. ½ react: Cd → Cd2+ + 2 eRed. ½ react: 2 e- + 2 H+ + ClO4- → ClO3- + H2O Cd Which species is the reducing agent? _______ Two electrons in each reaction means that 1 of the ox. ½ ½ reactions add to make the ClO4 reactions and +1 of the red. Which species is the oxidizing agent? _______ net: Cd + 2 H + ClO4 → Cd2+ + ClO3- + H2O - Cd provides the electrons, it is the reducing agent. Cl in ClO4 changes ox. # from +7 to +5; it is the oxidizing agent in the species perchlorate. 13. (10 pts) Fill in the blanks in the table (one column per element). Symbol 58 Ni 20 Ne ___________ Page 4 The atomic mass A in the 55 Mn ___________ superscript is the sum of # of prot. plus neutr. The # of e- in the neutral atom is equal to the # of protons. Chemistry 111 Sect. 3 Exam #1 October 5, 2000 Number of protons Number of neutrons Number of electrons in the neutral atom Name of element Name: _____________________ KEY _____________________ Student #: ______________________________________________ ___________ 28 30 ___________ 10 10 ___________ 25 30 28 ___________ nickel ___________ 10 ___________ neon ___________ 25 manganese ___________ 14. (10 pts) Do you agree with the following statement? Please try to answer in one or two sentences at most. “The number of grams in one mole of 20Ne is exactly 20.” No. The mole is defined based on exactly 12 g of 12C. All other elements and isotopes do not have exact integer atomic masses. Page 5 Chemistry 111 Sect. 3 Exam #1 October 5, 2000 Name: _____________________ KEY _____________________ Student #: ______________________________________________ Useful information NA = 6.02 × 10 23 molecules/mol d = m/V PERIODIC TABLE OF THE ELEMENTS IA IIA IIIB IVB VB VIB VIIB VIIIB IB IIB IIIA IVA VA VIA VIIA VIIIA 1 2 H He 1.008 4.003 3 4 5 Li Be B C N O F Ne 6.939 9.012 10.81 12.01 14.01 16.00 19.00 20.18 11 12 13 14 15 16 17 18 Na Mg Al Si P S Cl Ar 22.99 24.31 26.98 28.09 30.97 32.07 35.45 39.95 19 20 21 22 K Ca Sc Ti 39.10 40.08 44.96 37 38 39 Rb Sr 85.47 23 6 7 8 9 10 24 25 26 27 28 29 30 31 32 33 34 35 36 V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr 47.90 50.94 52.00 54.94 55.85 58.93 58.71 63.55 65.39 69.72 72.61 74.92 78.96 79.90 83.80 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te 87.62 88.91 91.22 92.91 95.94 (99) 101.1 102.9 106.4 107.9 112.4 114.8 118.7 121.8 127.6 126.9 131.3 55 56 57 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn 132.9 137.3 138.9 178.5 181.0 183.8 186.2 190.2 192.2 195.1 197.0 200.6 204.4 207.2 209.0 (209) (210) (222) 87 88 89 Fr Ra Ac (223) 226.0 227.0 Page 6 I Xe