Sample exam 1

Chemistry 123 Sample Exam 1 Sample Exam 1. The atomic mass of nitrogen is 14.007 g. Calculate the % abundances of the two isotopes of nitrogen if their isotopic masses are Isotopic mass % abundance 14.0031 15.001 2. Analysis shows that a certain ionic solid contains potassium plus one other element, a halogen X. Determine the identity of X from the mass percentages K: 52.4%, X: 47.6% What is the empirical formula of the compound? 3. Give the chemical formula for each of the following: a. xenon difluoride b. calcium hydrogen sulfite c. dinitrogen tetraoxide 4. Consider the compound, BaCrO4, a. Name this compound b. Determine its molar mass. c. Determine the %Ba in this compound. d. How many oxygen atoms are there in 0.50 mol of BaCrO4 molecules e. How many grams of BaCrO4 are in part (d) above? 5. Which sets of values are possible? a b c d e f Mass Number 19 235 53 32 14 40 Atomic Number 42 92 131 15 7 18 Number of Protons 19 92 131 15 7 18 Number of Neutrons 23 143 79 15 7 40 6. Gold can be dissolved from gold-bearing rock by treating the rock with sodium cyanide in the presence of the oxygen in air. Au (s) + NaCN (aq) + O2 (g) + H2O (l) → NaAu(CN)2 (aq) + NaOH (aq) – Once the gold is in solution in the form of the Au(CN)2 ion, it can be precipitated as the metal according to the following balanced equation: 2+ 2 Au(CN)2 (aq) + Zn (s) → Zn (aq) + 2 Au (s) + 4 CN- (aq) (a) How many moles of NaCN will you need to extract the gold from 1000 kg of rock if the rock is 0.019% gold? (b) How many kilograms of metallic zinc will you need to recover the gold from the Au(CN)2 obtained from the gold in the rock? (c) If the gold is recovered completely from the rock, and the metal is made into a cylindrical rod –3 15.0 cm long, what is the diameter of the rod? (The density of gold is 19.3 g cm .) 7. Name the following compounds: a. Na2Cr2O7 b. Cl2O7 c. MnO2 d. LiHSO3 8. Balance the following equations: a. NaHCO3 + SrCl2 + NaOH → SrCO3 + NaCl + H2O b. Ca(OH)2 + H3PO4 → H2O + Ca3(PO4)2 c. AgNO3 + H2SO4 → Ag2SO4 + HNO3 d. Cr + S8 → Cr2S3 9. Indicate which of the following compounds are likely to exist, based on your understanding of the periodic table and the formulas for polyatomic ions: Na3SO4 BaO KF3 CaI2 10. Name and assign oxidation numbers to all elements in the following substances: a. NaMnO4 b. KBrO3 11. Nitrogen gas can be prepared in the laboratory by the reaction of ammonia with copper(II) oxide to form nitrogen gas, elemental copper, and water. The reaction will give an 85.0% yield and you want to make 18.4 g of nitrogen gas. (a) What mass of ammonia must you start the reaction with? (b) What mass of copper(II) oxide do you need if you start with a 37.0% excess? 12. Write net ionic equations for the following reactions: (a) HNO3 (aq) + NaHCO3 (aq) → NaNO3 (aq) + CO2 (g) + H2O (l) → KCl (aq) + H2O (l) + NaOH (aq) → NaCl (aq) + NH3 (g) (b) HCl (aq) + KOH (aq) (c) NH4Cl (aq) + H2O (l) 13. A solution is made by mixing 200 mL 0.20 M NaOH with 100 mL 0.10 M HNO3. Find the number of moles of each ion and its concentration in the solution after reaction is complete. 14. It is found that 22.3 mL of 0.240 M NaOH is required to completely react with a 50.0-mL sample of vinegar, a solution of acetic acid in water. The net ionic equation for the reaction is HC2H3O2 (aq) + OH (aq) → C2H3O2 (aq) + H2O (l) Calculate the concentration of acetic acid in the vinegar. 15. What volume of each of the following acids will react completely with 50.00 mL of 0.200 M NaOH? (a) 0.100 M HCl (b) 0.0598 M HNO3 (c) 1.23 M HBr 16. Identify the following reactions as either dissolution, precipitation, redox, or acid-base: (a) N2O5 (g) + 2 NaOH (aq) → 2 NaNO3 (aq) + H2O (l) + 5 C (s) → P4 (g) + 5 CO2 (g) (c) 3 Ca(NO3)2 (aq) + 2 K3PO4 (aq) → Ca3(PO4)2 (s) + 6 KNO3 (g) (d) (NH4)2SO4 (aq) + Ba(OH)2 (aq) → BaSO4 (s) + 2 NH3 (g) + 2 H2O (l) (e) 2 KMnO4 (s) → K2MnO4 (s) + MnO2 (s) + O2 (g) (b) P4O10 (s) Sample Exam 1 1. 99.6% with mass 14.0031, 0.4% with mass 15.001 2. KCl 3. a. XeF2 b. Ca(HSO3)2 c. N2O4 24 2 4. a. barium chromate b. 253.321 g/mol c. 54.211% d. 1.2 × 10 atoms of O e. 1.3 × 10 g 5. b & e 6. a. 1.9 mol NaCN b. 0.032 kg Zn c. 0.91 cm 7. a. sodium dichromate b. dichlorine heptaoxide c. manganese(II) oxide d. lithium hydrogen sulfite 8. a. 1, 1, 1, 1, 2, 1 b. 3, 2, 6, 1 c. 2, 1, 1, 2 d. 16, 3, 8 9. BaO and CaI2 could exist 10. a. Na, +1, sodium; Mn, +7, manganese; O, -2, oxygen b. K, +1, potassium; Br, +5, bromine; O, -2, oxygen In both cases, alkali metals are always +1, oxygen is usually -2, so determine the middle oxidation number by algebraic means. 11. (a) 26.3 g NH3 (b) 253 g CuO 12. (a) H (aq) + HCO3 (aq) → CO2 (g) + H2O (l) + - → H2O (l) + OH (aq) → NH3 (g) + - (b) H (aq) + OH (aq) + - (c) NH4 (aq) - - + H2O (l) + + 13. 0.010 mol NO3 in sol’n, [NO3 ] = 0.033 M; 0.040 mol Na in sol’n, [Na ] = 0.13 M; - - 0.030 mol OH in sol’n, [OH ] = 0.10 M 14. [HC2H3O2] = 0.107 M 15. (a) VHCl = 0.100 L (b) VHNO3 = 0.167 L 16. (a) acid-base (b) redox (c) precipitation −3 (c) VHBr = 8.13 × 10 L (d) acid-base (e) redox