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1 HOMEWORK #2: ANSWER KEY Multiple Choice [Please note that some questions have been deliberately removed and so you should not worry about any discontinuity in the numbering] Identify the choice that best completes the statement or answers the question. ____ 1. If the relative atomic mass of 56 Fe is 55.935 (on the 12 C scale), what is the mass of one 56 Fe atom? a. 3.37 ! 10 –25 g b. 1.66 ! 10 –24 g c. 7.74 ! 10 –24 g d. 9.29 ! 10 –23 g e. 3.37 ! 10 –22 g Answer: d One mole (i.e. 6.022 x 10 23 ) of Fe atoms has a mass of 55.935 g. Therefore, to get the mass of one ATOM, divide 55.935 grams/mole by 6.022 x 10 23 which is 9.29 ! 10 –23 g . ____ 2. Which has the greatest number of hydrogen atoms? a. 10 20 hydrogen atoms b. 100 g of water c. 5 g of an unknown compound d. 20 g of hydrogen gas e. 100 g of a substance that is 2% H by mass Answer: d Choice (a) clearly specifies the number of hydrogen atoms to be 10 20 . In choice (b), we first need to convert 100 g of water to the number of moles of water, which is 100/18 = 5.555. There are 2 moles of hydrogen per 1 mole of water so there are 11.11 moles of hydrogen. 11.11 multiplied by Avogadro’s number (i.e. 6.022 x 10 23 ) gives us the number of hydrogen atoms which is 6.69 x 10 24 hydrogen atoms. Choice (c) can be discounted because choice (d) says 20 grams of hydrogen gas so choice (c) has less hydrogen atoms than (d). 20 grams of hydrogen gas (i.e. H 2 ) contains 20/2 = 10 moles of H 2 molecules. Since each molecule of H 2 contains two hydrogen atoms, 10 moles of H 2 molecules contains 20 moles hydrogen atoms. 20 moles x Avogadro’s # gives us 1.2 x 10 25 which is the most hydrogen atoms and the right answer. Choice (e) is 2% of 100 grams by mass which is 2 grams which is less than choice (d).

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2 ____ 3. Which of these carbohydrates has the largest molecular mass? a. threose (C 4 H 8 O 4 ) b. ribose (C 5 H 10 O 5 ) c. glucose (C 6 H 12 O 6 ) d. sucrose (C 12 H 22 O 11 ) e. raffinose (C 18 H 32 O 16 ) Answer: e The molecular mass for a compound of formula is C x H y O z is (12x + 1y + 16z). By inspection, it is obvious that this number is greatest for C 18 H 32 O 16 . ____ 4. What is the relative molecular mass of the compound trinitrotoluene, C 7 H 5 N 3 O 6 (on the 12 C scale)? a. 43.03 b. 205.13 c. 215.13 d. 227.13 e. 278.03 Answer: d Carbon has a mass of 12. Hydrogen has a mass of 1. Nitrogen has a mass of 14. Oxygen has a mass of 16. To get the molecular mass, you multiply the amount of each atom in the molecule by its atomic mass and take the sum of those masses. Therefore, C 7 H 5 N 3 O 6 has a molecular mass of 12(7) + 1(5) + 14(3) + 16(6) = 227. Choice d is correct. Note that the value given in d (227.13) is to two decimal places, but it is not necessary to use atomic masses to this precision to get the required answer in this case. ____ 5.
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