Ex_1AKey Fall2015.pdf

For grading problem score 1 21 2 15 3 20 4 10 5 10 6

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(For Grading) Problem Score 1 (21) 2 (15) 3 (20) 4 (10) 5 (10) 6 (12) 7 (12) TOTAL

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NAME:_________________________________ © 2015 L.S. Brown A3 2. The density of titanium is 4.51 g/cm 3 , and here we will consider a particular titanium sphere that weighs 20.19 lb. (6 pts) (a). How many titanium (Ti, Z = 22) atoms does this sphere contain? 20.19 lb × 454 g 1 lb × 1 mole Ti 47.88 g Ti × 6.022 × 10 23 atoms 1 mol Ti = 1.15 × 10 26 atoms (9 pts) (b). What is the radius of the sphere in inches? (The volume of a sphere is V = 4 3 π r 3 , and various unit conversion factors are given on the front page of your exam. Show ALL of your work clearly; do not use any unit conversion functions that may be available on your calculator.) 20.19 lb × 454 g 1 lb × 1 cm 3 4.51 g Ti = 2032 cm 3 2032 cm 3 × 1 in 2.54 cm 3 = 124.0 in 3 r 3 = 3 V 4 π = 3(124.0 in 3 ) 4 π = 29.6 in 3 r = (29.6 in 3 ) 1/3 = 3.09 in
A4 © 2015 L.S. Brown 3. (6 pts) (a). Ammonia (NH 3 ) can react with O 2 gas to form nitrogen dioxide (NO 2 ) and water. Write a properly balanced chemical equation for this process. Please write your final balanced equation neatly in the box below! (8 pts) (b). Consider the reaction: 3 NaBH 4 + 4 BF 3 2 B 2 H 6 + 3 NaBF 4 Suppose that 20 moles of NaBH 4 and 24 moles of BF 3 are combined and allowed to react to completion. Fill in the blanks below to show the expected composition of the resulting mixture. __ 2 __ mol NaBH 4 __ 0 __ mol BF 3 __ 12 _ mol B 2 H 6 _ 18 __ mol NaBF 4 (6 pts) (c). The compound shown below is emitted by ladybugs when they are threatened or squashed. Write its correct molecular formula. _____ C 8 H 12 N 2 O _ N N O 4 NH 3 + 7 O 2 à 4 NO 2 + 6 H 2 O

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NAME:_________________________________ © 2015 L.S. Brown A5 (10 pts) 4. A particular compound contains 79.95% bromine by mass and has the formula XBr 2 , where X is an unknown element. Find the identity of element X. (You must show sufficient work to justify your choice!) We can find the molar mass of the compound, and from that the molar mass of element X, which will let us identify it.
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