Chemistry 1A Fall 2009 Midterm Exam 1 September 15 2009 90 min closed book Name SID GSI Name The test consists of 8 short answer questions and 15 multiple choice questions Put your written answers in the boxes provided Answers outside the boxes may not be considered in grading Write your name on every page of the exam Question Multiple Choice 1 15 Question 16 18 Page Points 2 5 45 6 6 Question 19 21 7 9 Question 22 8 8 Question 23 9 12 Total 80 Score Useful Equations and Constants PV nRT n2 P a 2 V V KE nb nRT 1 2 mv 2 Ptotal PA PB XAPA XBPB N0 6 02214 x 1023 mol 1 T K T C 273 15 R 0 0821 L atm K 1 mol 1 1 atm 760 mm Hg 760 torr 1 bar Name GSI Multiple choice Circle the BEST answer and bubble the choice on your scantron form 1 Which of these ions is least likely to form A B C Ba2 P3C4 D Cl3 2 Which of these substances is most likely to be a liquid at room temperature A B C D C3H8O ZnS Mo Se 3 Why does the gas pressure of N2 molecules in a container of fixed volume increase as the gas temperature increases from 25 C to 100 C A The gas molecules hit the walls with more energy B The gas molecules break apart into atoms C The gas molecules hit the walls more often D Both A and C 4 Air bags have been used to land spacecraft on Mars A volume of 500 L is required for the air bag You predict that the pressure on Mars will be 5 5 x 10 3 atm and the temperature will be 258 K How many moles of gas do you need to inflate the airbag A 0 13 B 21 C 25 D 55 5 Assume you have a mixture of He light circles and Xe dark circles at 300 K Which drawing best represents the mixture A B C D Page 2 of 9 Name GSI 6 Which of these substances has the smallest bond angle A H2O B Cl2O C NH3 D PbCl2 E O3 7 Which of these substances is the most polar molecule A SO3 B SF2 C H2S D NH4 E CF4 8 Which of these atoms has the largest ratio of effective nuclear charge to radius Zeff r on the valence electrons A Li B Na C C D F E Cl 9 Which of these elements form a gaseous molecule with the formula XF2 Where X is the element A Mg B S C Cl D N E B 10 A stopper is placed on several flasks with the contents as described below Before putting on the stopper liquid is poured into some of the flasks Which sealed container has the highest total gas pressure A 200 mL flask with air and 190 mL water because the gas volume is the smallest B 200 mL flask with air and 50 mL ethanol C2H6O because ethanol has a higher vapor pressure than water C 500 mL flask with air and 400 mL water because the liquid volume is the largest D 500 mL flask with air because the gas volume is the largest E 500 mL flask with air If liquid is present the air dissolves in the liquid thereby lowering the gas pressure Page 3 of 9 Name GSI Name Structure Formula Molar Mass g mol a bar L2 mol 2 b L mol 1 Methanol CH3OH 32 042 9 476 0 0659 Hydrazine N2H4 32 045 8 46 0 0462 Silane SiH4 32 118 4 38 0 0579 11 The van der Waals equation provides corrections for the assumptions made by the ideal gas law in order to apply to real gases Data for three gases can be found in the table above Which of the gases listed would you expect to have the lowest boiling point and why A Methanol because it has the biggest volume B Methanol because it can form hydrogen bonds between molecules C Hydrazine because it has a low mass and is nonpolar D Hydrazine because it has the smallest volume E Silane because it has the weakest attractions 12 Substances can be spread over a surface with a paintbrush Which of these substances could be spread by a paintbrush and why A Lead Pb because it is ductile B Silicon dioxide SiO2 because it sublimes C Propanol C3H8O because it has a high vapor pressure D Potassium nitrate KNO3 because the ions are attracted to the surface E Octane C8H18 because the molecules are not strongly attracted to each other 13 Acetic acid shown below tastes sour and smells putrid Which one of the substances O below would also taste sour H O O A B O C D Page 4 of 9 Name GSI 14 You have an empty flask at 25 C connected through a narrow piece of tubing to water in beaker You heat the flask with a hot flame and then place a cork on the top of the flask The flame is removed and the system is allowed to cool back down to 25 C Describe what you observe once the flask has cooled down to 25 C A B C D E There are gas bubbles in the water because the air in the flask expands upon heating The water level in the beaker rises because of hydrogen bonding Nothing The flask started at room temperature and ends at room temperature The cork pops off because the air pressure increases Water flows into the flask from the beaker because the pressure of the gas inside the flask decreases 15 A certain gas made up of molecules containing only carbon and hydrogen occupies 22 4L at 546 K and 0 5 atm An analysis of the gas shows that it is composed of 1 mole of carbon atoms and 2 moles of hydrogen atoms What is the molecular formula of the particles in the gas A CH2 B C2H4 C CH4 D C4H8 E C2H2 Page 5 of 9 Name GSI 16 The Maxwell Boltzmann speed distribution for 1 atm nitrogen gas N2 at 300 K is shown in the graph below Draw the speed distribution for 50 atm of carbon dioxide gas CO2 at 300 K Assume that the total number of molecules in each sample is identical 17 A sketch of a balloon at 1 atm pressure is shown below Imagine a scuba diver takes the balloon underwater to a depth where the total pressure on the balloon is 2 atm Sketch the balloon at 2 atm pressure Be specific about the relative size and shape 18 Draw all possible structural formulas for C2H6O that represent molecules with distinct properties Satisfy the octet rule Page 6 of 9 Name GSI 19 Explain why methanol CH4O has a higher boiling point than formaldehyde CH2O Provide Lewis dot structures to justify your answer 20 White phosphorus exists as P4 with the molecule in the shape of a tetrahedron as shown below …