Name: Prelab Homework 7 Each part is worth 1 point, or as indicated. 1. A certai

Question

Name: Prelab Homework 7 Each part is worth 1 point, or as indicated. 1. A certain compound has the following elemental analysis (percent composition by mass), where "X" is an unknown element (N, O, CL, or Br): 96 C 74.931% H 12.5% % X 12.48 Its IR spectrum and mass spectrum are given below. Answer the questions on the next page. While you do not need to fully analyze all of this data or determine the exact structure, you may wish to use different spectral details to help determine certain functional groups or other substructures. 100 90 60 20 5o 75 300 125 25

Explanation / Answer

a) The absence of -C=O at around 1700 cm-1 rules out ketone, carboxylic acid and amides. Strong peak around 3300 cm-1 could be due to hydroxyl –OH group. Further, a corresponding C-O peak at around 1000 cm-1 could be due to an alcohol. Hence, the unknown element X could be Oxygen.

b) Given, in the question

C is 74.93 %
H is 12.59 %
O is 12.48 %

Let us assume that100 g of the compound is present. Hence, the elements present in grams are:

C 74.93 g
H 12.59 g
O 12.48 g

The masses of each element can be converted into moles:

C 74.93 g / 12 = 6.24
H 12.59 g / 1 = 12.59
O 12.48 g / 16 = 0.78

To get the smallest whole-number ratio, divide other by the lowest,

C 6.24 / 0.78 = 8
H 12.59 / 0.78 = 16
O 0.78 / 0.78 = 1

Thus, the empirical formula is C8H16O

c) The mass of the molecular ion as evident from the given mass spectrum is 128 u.

d) To determine the molecular formula from the empirical formula i.e., C8H16O

"EFW" 12x8 + 1x16 + 1x16

             96 +16 + 16 = 128

Hence, the molecular formula is also C8H16O.

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