Empirical Biomass Formula: A sample of a biological culture is submitted to a la

Question

Empirical Biomass Formula: A sample of a biological culture is submitted to a laboratory for analysis of the percentage by weight of each of the major elements present in the organic portion. The laboratory evaporates the sample to dryness and then places it in an oven overnight at 150° C to drive off all water present. The organic portion of the residue remaining is then analyzed after which the sample is burned in a Muffle furnace at 550°C to determine the weight of the ash remaining. The composition of the cells by weight is found to be 47.4% C, 5.5% H, 25.6% 11.2% N, and 9% ash. Calculate the empirical formula for the cells and normalize the formula to nitrogen and then to carbon. Calculate the molecular weight and the COD' of the biomass. 1. 2.

Explanation / Answer

Part 1

Basis - 100 g of cell composition

Mass of C = 47.4 g

Moles of C = mass/molecular weight

= 47.4/12 = 3.95 moles

Mass of H = 5.5 g

Moles of H = 5.5/1 = 5.5 moles

Mass of O = 25.6 g

Moles of O = 25.6/16 = 1.6 moles

Mass of N = 11.2 g

Moles of N = mass/molecular weight

= 11.2/14 = 0.8 moles

Total moles = 3.95 + 5.5 + 1.6 + 0.8 = 11.85 moles

Mol fraction of C = moles of C/total moles

= 3.95 /11.85 = 0.333

Mol fraction of H = 5.5/11.85 = 0.464

Mol fraction of O = 1.6/11.85 = 0.135

Mol fraction of N = 0.8/11.85 = 0.0675

Normalize the formula to N

Divide each mol fraction by 0.0675

C = 4.9

H = 6.8

O = 2

N = 1

Empirical formula for cells = C4.9 H6.8 O2 N

Part 2

Molecular weight = (12*4.9) + (1*6.8) + (16*2) + (1*14)

= 111.6

COD' of the biomass = (2 x 4.9 + 0.5 x 6.8 - 2 - 1.5)*16

= 155.2

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