hmwk3ans

hmwk3ans - Chemistry 209 Chapter 3 Homework#3 answers 14 P4...

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Chemistry 209 Homework #3 answers Chapter 3: 14 P 4 units have molecular weight 4 × 30 . 9738 = 123 . 8952. The density of the P 4 unit is given as 1.823 g/cm 3 and we are given a cylinder 6.50 cm in length and 1.22 cm in diameter. The volume of a cylinder is given by V = πr 2 h = π d 2 4 h for our sample V = 3 . 14159 × 0 . 61 2 × 6 . 50 cm 3 = 7.598 cm 3 ; and given the density d = 1 . 823 we know that we have a total mass of P 4 m = V d = 7 . 598 cm 3 × 1 . 823 g cm 3 = 13 . 852 g (a) Given the molecular weight calculated above, that corresponds to n moles of P 4 given by n = 13 . 852 grams sample × 1 mole 123 . 8952 g = 0 . 1118 moles P 4 (b) This one is easy; one mole contains N A = 6 . 0221 × 10 23 molecules; so 0.1118 moles contains 0 . 1118 × 6 . 0221 × 10 23 = 6 . 733 × 10 22 P 4 molecules. 18 Given the compound Ge[S(CH 2 ) 4 CH 3 ] 4 , we are asked to figure out what we have. First, let’s start with a picture of the molecule, or at least an artist’s conception of what might be the molecule: CH 3 CH 2 CH 2 CH 2 CH 2 S | CH 3 CH 2 CH 2 CH 2 CH 2 S Ge SCH 2 CH 2 CH 2 CH 2 CH 3 | SCH 2 CH 2 CH 2 CH 2 CH 3 (a) How many atoms are found in one formula unit? Well, there are four equivalent groupings of atoms, each containing 1 S atom, 4 CH 2 groups, and a single CH 3 group. By my count that corresponds to 1+4 × 3+4 = 17 atoms in each supgrouping; there are four of them, plus the central Ge atom, fo I count a total of 69 atoms per formula unit. (b) How many C and/or H atoms? They are found in CH 2 and CH 3 groups; there are 4 × 4 of the former, and 4 × 1 of the latter. So the total count of carbon atoms is 20; the total count of hydrogen atoms is 44; and the ratio of C atoms to H atoms is 20:44; or in simplified numbers, 5:11. (c) The ratio by mass of Ge to S; each Ge weighs 72.61 (on average, assuming that we have a natural abundance distribution of isotopes and that there are suﬃcient atoms present so that the average reasonably represents any distribution; and there are 4 S atoms per Ge, each of which weighs (again, on average) 32.066. So the ratio of Ge mass to S mass

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