APC Spring Break Take-Home Exam Instructions

Transcription

1 APC Spring Break Take-Home Exam Instructions Complete all exam questions on separate paper. Show all work to receive credit. Partial credit will be awarded! Staple all papers together. Do NOT include the exam questions. If you are unable to develop an answer to a question in person, you will not be given credit for the original response. o In other words: study groups are a great idea but only if you understand what you ultimately submit as your own work. Completing this take-home exam is not sufficient preparation for the 3 rd quarter exam. Some topics are only superficially covered; others are noticeably missing. You have been warned! I will be available via throughout spring break to answer general, conceptual questions. Please give me 48 hours to respond. Due at the beginning of class on Wednesday, April 3 rd. Late work will not be accepted.

2 APC Spring Break Take-Home Exam 1. Decomposition of g of a pure solid compound produces 4.06 g of nitrogen gas, g of water, and a solid metal oxide. The metal oxide is found to contain 68.42% chromium. a. What is the formula for the metal oxide produced? b. What is the oxidation number of chromium in the oxide? c. What is the formula for the unknown pure solid compound? d. Based on your answer to part c, represent the formula of this compound as a ratio of two common polyatomic ions. e. Write and balance a chemical equation for the decomposition reaction. 2. Muriatic acid is a solution of hydrochloric acid used in construction to clean bricks and etch concrete. A titration was performed to determine the percent of hydrochloric acid in a bottle of muriatic acid. a. First, the concentration of a standard sodium hydroxide solution was determined by titration. It was found that g of potassium hydrogen phthalate, KHC 8 H 8 O 4, an acid, required ml of NaOH solution to reach the end point. The neutralization reaction is shown below. Calculate the concentration of NaOH. b. A ml sample of diluted muriatic acid required ml of standard NaOH solution to reach the end point. Calculate the concentration of the diluted muriatic acid solution. c. The diluted muriatic acid in (b) was made by diluting ml of concentrated muriatic acid to a total volume of ml. What was the concentration of the original sample of muriatic acid? d. The density of the original muriatic acid solution was found to be 1.15 g/ml. What is the percent by mass of HCl in the original muriatic acid solution? e. Laboratory grade hydrochloric acid is 37.0% HCl and has a density of g/ml. Calculate the molarity of laboratory hydrochloric acid.

3 3. Two experiments are carried out to determine the molecular formula of an organic acid containing only carbon, hydrogen, and oxygen. First, combustion of grams of the acid produces g of carbon dioxide and g of water. a. How many moles of oxygen gas are needed for the combustion reaction? b. What is the empirical formula of the compound? In a second experiment, titration of g of acid requires ml of M NaOH solution to reach a suitable end point. c. Calculate the molar mass of the acid. Hint: by what do you have to divide the given grams of acid in order to get molar mass? d. What is the molecular formula of the acid? e. Write and balance an equation for the complete combustion of the acid. 4. Aqueous nickel (II) sulfate is added to a solution of sodium hydroxide. a. Write a balanced net ionic equation for this reaction. b. Would you expect any of the reactants and/or products to be colored? If so, which? Be specific. 5. Fluorine combines with sulfur to form SF 2, SF 4, and SF 6. a. Draw a Lewis structure for SF 2. b. Identify the hybridization of the sulfur in SF 4. c. State the geometry of SF 6. d. Predict the F-S-F bond angles for each of the three compounds: SF 2, SF 4, and SF 6. e. Predict which, if any, of the three sulfur compounds is polar. 6. Consider an atom of titanium. a. Write the complete electron configuration for a titanium atom in the ground state. b. Write the complete electron configuration of a titanium cation with a charge of +2. c. Write one possible set of four quantum numbers for one of the valence electrons of the titanium ion in part (b). d. Write the chemical formula for titanium (IV) oxide, which also goes by the common name titanium dioxide.

4 7. Consider the elements in Period 3 in order of increasing atomic number. a. First ionization energy generally increases across a period. Explain this in terms of effective nuclear charge and number of energy levels. b. Despite the trend in part (a), the first ionization energy of aluminum is lower than the first ionization energy of magnesium. Explain this phenomenon in terms of orbitalfilling. c. Predict the relative values of the second ionization energies of sodium and magnesium. Explain your reasoning. d. Why is the first ionization energy of potassium lower than that of sodium? 8. Four visible lines of the atomic emission spectrum of hydrogen have wavelengths of 410, 434, 486, and 656 nm. a. Calculate the frequency associated with the 410 nm line. b. Calculate the energy of a single photon associated with the 434 nm line. Calculate the energy of a mole of photons associated with the 486 nm line. c. Explain the origin of the four colored lines of the hydrogen spectrum. 9. A mixture of mol each of hydrogen gas, oxygen gas, and liquid water are present in a liter container at 25 C. The equilibrium vapor pressure of water at 25 C is torr. a. Calculate the total pressure inside the flask. b. Explain which gas has the i. Highest average kinetic energy ii. Highest average velocity of molecules iii. Highest density c. When the mixture is sparked, a reaction ensues that consumes one of the reactants entirely, and the resulting mixture is allowed to come to equilibrium at 80 C. i. Write and balance a chemical equation to describe the reaction. ii. Which is the limiting reactant? Show your work. iii. Calculate the total pressure in the flask after the reaction is complete. The water vapor pressure at 80 C is 355 torr. iv. Calculate the number of moles of water present as vapor when the reaction is complete.

5 10. Consider the following 0.10 M solutions: iron (III) chloride, copper (II) nitrate, and hydrogen sulfide. a. Describe the color of each solution. b. Which solution will have the highest conductivity? Explain. c. Which solution will have the highest boiling point? Explain. d. Which solution will have the highest freezing point? Explain. e. Which solution will have the lowest vapor pressure? Explain. f. Identify two solutions that, when mixed, will produce a detectable reaction. Write a net ionic equation for this reaction. 11. The unknown compound dissolves readily in chloroform, CHCl 3. The freezing point of a solution prepared by mixing grams of CHCl 3 and grams of the unknown compound is C. The molal freezing point depression constant of CHCl 3 is 4.68 C/molal and its normal freezing point is C. a. Calculate the molality of the unknown compound in the chloroform solution. b. Calculate the molar mass of the unknown compound. c. Determine the molecular formula of the unknown compound. d. Write and balance an equation for the combustion of the unknown compound. 12. When solid ammonium chloride is added to liquid water, the solid dissolves completely. a. What forces of attraction are present in pure solid ammonium chloride? b. What forces of attraction are present in pure liquid water? c. What intermolecular forces of attraction exist in an aqueous solution of ammonium chloride that do not exist in either pure ammonium chloride or pure water? 13. Consider 3.00 liters of a saturated solution of calcium hydroxide. The ph of the saturated solution is a. Write the chemical equation for the dissociation of solid calcium hydroxide in water. Assume calcium hydroxide dissociated completely. b. Calculate the concentration of hydroxide ion in a saturated solution of calcium hydroxide. c. Calculate the solubility, in g/l, of calcium hydroxide in water.