Chem Hughbanks Exam 1 - Spring 2013

Transcription

1 Chem Hughbanks Exam 1 - Spring 2013 UIN # Section 502 Exam 1, Version # A On the last page of this exam, you ve been given a periodic table and some physical constants. You ll probably want to tear that page off to use during the exam you don t need to turn it in with the rest of the exam. The exam contains 10 problems, with 5 numbered pages. You have the full 75 minutes to complete the exam. Please show ALL your work as clearly as possible this will help us award you partial credit if appropriate. Even correct answers without supporting work may not receive credit. You may use an approved calculator for the exam, one without extensive programmable capabilities or the ability to store alphanumeric information. Print your name above, provide your UIN number, and sign the honor code statement below. On my honor as an Aggie, I will neither give nor receive unauthorized assistance on this exam. SIGNATURE:

2 (1) (7 points) Consider the complete neutralization of 1 L of 0.1 M HCl with 1 L of 0.1 M NaOH by combining the two solutions. Assuming the reactant solutions and product solution have approximately the same density as water, which of the following statements is true? There may be more than one correct choice; enter ALL the correct letters, in alphabetical order. (a) In the total of both the reactant solutions and in the product solution, 0.1 moles of sodium ions are present. (b) The combined mass of the product solution is the same as the combined mass of the reactant solutions. (c) In all the products, the total number of atoms exceeds the total number of atoms in all the reactants. (d) The approximate concentration of chloride ions in the original HCl solution and in the product solution is the same. (e) In the total of both the reactant solutions and in the combined product solution, the total number of electrons is the same. (For grading) Scores 1 /7 2 /7 3 /10 4 /8 5 /10 6 /10 7 /10 8 /12 9 /16 10 /10 Tot. /100 Ans. 1 a, b, e (2) (7 points) In the blanks provided, place these gases in order of increasing average molecular speed at 25 C: (A) N 2 (B) Ar (C) CH 3 Cl (D) CH 4 (E) CF 4. E < C < B < A < D Insert the letters for each choice in the spaces. Be legible. This is an all-or-nothing question, the order must be entirely correct to get any credit. (3) (10 points) In the first row of blanks beneath each of the following compounds, classify them as follows: weak acid (wa), strong acid (sa), weak base (wb), strong base (sb), or as a salt (s) that is neither acid or basic. In the second row of blanks, classify the same compounds as a weak electrolyte (we), strong electrolyte (se), or nonelectrolyte (ne). HClO 4 CH 3 NH 2 KOH NaCl CH 3 COOH sa wb sb s wa se we se se we 1

3 (4) (8 points) The following full sets of potential compounds are made up of common cations and anions. Which full sets are most likely to actually exist? (All 3 compounds in a set must be sensible to qualify.) There may be more than one correct choice; enter ALL the correct letters, in alphabetical order. (a) Rb(SO 4 ) 2, CaCO 3, ZnClO 4 (b) NH 4 PO 4, NaCO 3, KNO 3 (c) Ca 3 (PO 4 ) 2, Ag 2 SO 4, NH 4 NO 3 (d) K 2 SO 4, KClO 4, Ca(OH) 2 (e) (NH 4 ) 2 PO 4, Li 2 CO 3, Ca 3 (PO 4 ) 2 Ans. 4 c, d, (5) (10 points) Calcium hydroxide can be dissolved in phosphoric acid to form calcium phosphate and water. (i) (5 pts) Balance the equation below by filling in the blanks with coefficients: 3 Ca(OH) H 3 PO 4 1 Ca 3 (PO 4 ) H 2 O (ii) (5 pts) What volume (in ml) of 1.32 M H 3 PO 4 is needed to react with 12.0 g of Ca(OH) 2? (Write the letter of the correct answer below.) (a) ml (b) 81.8 ml (c) ml (d) ml (e) ml Ans. 5 (ii) b (6) (10 points) Write the letters for the endings of the following sentence that form a true statement in the blank provided. (Enter ALL the correct letters, in alphabetical order.) If equal masses of He gas and Ar gas are mixed together in a flask, (a) the average velocity of the Ar atoms is 10 times smaller than the average velocity of the He atoms. (b) the helium partial pressure is 10 times the argon partial pressure. (c) the number of collisions of the helium atoms against the walls of the flask is 10 times the number of collisions of the argon atoms against the walls of the flask. (d) the total kinetic energy of all the atoms in the flask will be 11 times the total kinetic energy of just the argon atoms alone. (e) raising the temperature of the gas mixture from 100 C to 73 C will double the total kinetic energy of all the atoms in the flask. Ans. 6 b, d, e 2

4 Questions 7-11 show all work (48 pts) (7) (10 points) The average (root-mean-square) molecular speed for a common diatomic (two atoms per molecule) gas at 273 K is km/hour. What is the molar mass (in g/mol) of this common gas? Show all work. ½ Mv 2 = 3 / 2 RT v = ( km/h)(1000 m/km)/(3600 s/h) = m/s M = 3RT/v = 3(8.314 J/mol K)(273 K)/( m 2 /s 2 ) = kg/mol = 32 g/mol (8) (12 points) Diborane, B 2 H 6, and related compounds were briefly considered as potential rocket fuels in the 1950s. A representative reaction for this class of molecules is that between B 2 H 6 and O 2 to form B 2 O 3 and H 2 O. (a) (5 points) Write a balanced equation for the reaction between B 2 H 6 and O 2. B 2 H O 2 B 2 O H 2 O (b) (7 points) Compute the masses of B 2 O 3 and H 2 O formed if g of B 2 H 6 reacts as completely as possible with g O 2. (210.4 g O 2 )/(32 g/mol) = mol O 2 (55.34 g B 2 H 6 )/(27.6 g/mol) = mol B 2 H 6 B 2 H 6 is the limiting reagent, so mol B 2 O 3 and mol H 2 O will form (2.005 mol B 2 O 3 )(69.6 g/mol) = g B 2 O 3 (6.015 mol H 2 O)(18.0 g/mol) = g H 2 O 3

5 (9) (16 points) Under appropriate conditions potassium chlorate, KClO 3, might be used to generate pure oxygen for use in breathing apparatus. When heated with a catalyst, it generates innocuous potassium chloride, KCl, and oxygen:. 2 KClO 3 2 KCl + 3 O 2 Note: Catalysts are used to speed the rate of a reaction, but are not consumed in reactions therefore all the catalyst remains after the reaction has ended. (a) (2 points) Using the blanks provided, balance this equation with the smallest possible integer coefficients. A g mixture of KClO 3 and a catalyst were heated together and the above reaction occurred in quantitative (100 %) yield. All the oxygen gas was collected in a balloon at 27 C and 1.0 atm. pressure. (b) (10 points) The KCl formed in the reaction is completely soluble in water and the catalyst is completely insoluble in water. The catalyst/kcl mixture present after this reaction was thoroughly washed with water to give 80 ml of a solution that was analyzed for its chloride content with the result: [Cl ] = M. What was the volume of O 2 collected in the balloon? mol Cl = (0.080 L)( mol/l) = mol mol O 2 = 3 / 2 ( mol) = mol Vol O 2 = so ( mol)( L-atm/mol K)(300 K)/(1.0 atm) Vol O 2 = L = 50 ml (c) (4 points) What was the mass percent of KClO 3 in the original KClO 3 /catalyst mixture? Started with mol KClO 3 mass KClO 3 = ( mol)( g/mol) = g % KClO 3 = ( g)/(0.200 g) 100% = 82.8% 4

6 (10) (10 points) Shown below is the molecular structure of adrenaline, which is a stimulant produced by the body in response to various stimuli. (a) (6 pts) Give the molecular formula for adrenaline. C 9 H 13 NO 3 (b) (4 pts) Assuming your answer is correct; give the molar mass of adrenaline g/mol 5

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