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Logic Puzzle #26: Elemental Analysis to Empirical Formula … Solution

The results for calculating the empirical formula for an unknown compound using data from elemental analysis are presented.

An elemental analysis was collected for an unknown organic compound. A 0.30 g sample of the unknown was burned to completion and it gave 59.6 % C, 6.9 % H and 8.7 % N. The results are summarized in the table below. The calculation for the unknown organic compound assumes that the missing % composition is oxygen.

Atom moles = Composition * Sample Mass / Atomic Weight (1) or

Atom moles = Composition / Atomic Weight (2)

Atom

Composition (%)

Sample Mass (g)

Atomic Weight (g/mol)

Formula 1 (mol)

Mol Ratio (/ 0.186)

Mol Ratio (X 2)

C

59.6

0.30

12.0107

1.489

8.01

16.01

H

6.9

0.30

1.0079

2.057

11.06

22.12

N

8.7

0.30

14.0067

0.186

1.00

2.00

O

24.8

0.30

15.9994

0.465

2.50

5.00

Using formula 1, the Mol ratios are calculated. Dividing by the smallest value (0.186) and then multiplying by 2, the empirical formula for the unknown is C16 H22 N2 O5.

To determine the molecular formula for the unknown, additional information such as the molecular weight is necessary.

Thank you Walter and Canageek for your comments.