Molecular Formulae Problems Directions: Show all work for this problem. You may

Question

Molecular Formulae Problems Directions: Show all work for this problem. You may want to write your answers on a piece of notebook paper. Answer all questions by showing all mathematical work and/or interpreting all major peaks in the IR and all peaks in the 'H-NMR spectra. You may wish to do complete your assign- ment on another sheet of paper. 1. A researcher analyzed an unknown solid, extracted from the bark of spruce trees, to determine its percentage composition. An 11.32 mg sam- ple was burned in a combustion apparatus. and water (5.82 mg) were collected and weighed. a. From the results of this analysis, calculate the percentage composition of the unknown solid. b. Determine the empirical formula of the unknown solid.

Explanation / Answer

Since only CO2 and H2O were produced, this means that the unknown solid is a hydrocarbon.

Moles of CO2 produced = Mass/MW = 0.02487/44 = 0.000565

Moles of C = Moles of CO2 = 0.000565

So,

Mass of C = Moles*MW = 0.000565*12 = 0.00678 g

Moles of H2O = Mass/MW = 0.00582/18 = 0.000323

Moles of H = 2*0.000323 = 0.000646

Mass of H = 0.000646*1 = 0.000646 g

So,

Mass of O = Mass of sample - (Mass of C + Mass of H) = 0.01132-(0.00678+0.000646) = 0.003894 g

So,

% composition is:

C = (0.00678/0.01132)*100 = 59.9%

H = (0.000646/0.01132)*100 = 5.7%

O = 100-(59.9+5.7) = 34.4%

(b)

Molar composition is:

C = 0.000565

H = 0.000646

O = 0.003894/16 = 0.0002434

Since O has the smallest number of moles, so we divide all the values by this number.

C = 0.000565/0.0002434 = 2.32

H = 0.000646/0.0002434 = 2.65

O = 1

Multiplying by 2,

C = 4.64

H = 5.3

O = 2

So the empirical formula is C5H5O2.

Hope this helps !

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