Frequently Asked Questions
How does combustion analysis determine a molecular formula?
Burning an organic compound in excess oxygen converts all carbon to CO2 and all hydrogen to H2O. By weighing the absorbed CO2 (in a CO2 trap) and H2O (in a desiccant tube), the masses of C and H in the sample are calculated. Any remaining mass is assumed to be oxygen (or other elements analyzed separately). The mole ratios give the empirical formula.
What is the empirical formula and how is it different from the molecular formula?
The empirical formula gives the simplest whole-number ratio of atoms. The molecular formula gives the actual count of atoms per molecule. Combustion analysis gives the empirical formula; you need the molar mass (from mass spectrometry or the ideal gas law) to find the molecular formula. Example: empirical CH2O (MW 30) could be formaldehyde (CH2O), acetic acid (C2H4O2), glucose (C6H12O6) etc.
Why is oxygen calculated by difference in combustion analysis?
Oxygen from the sample combines with carbon and hydrogen during combustion, so it ends up split between CO2 and H2O and cannot be separately collected. Instead: mass O = sample mass - mass C - mass H - mass N (from separate Dumas analysis). This subtraction works because all other elements are accounted for, and the remainder must be oxygen.
Can combustion analysis distinguish glucose from formaldehyde?
No, not on its own. Both glucose (C6H12O6) and formaldehyde (CH2O) reduce to the same empirical formula, CH2O, so burning either one gives CO2 and H2O in the same 1:2:1 C:H:O mole ratio. Per gram of sample the products are identical. To tell them apart you need the molar mass from a separate measurement: formaldehyde is 30 g/mol (n = 1, so the molecular formula is CH2O itself), while glucose is 180 g/mol (n = 180/30 = 6, giving C6H12O6). Acetic acid, C2H4O2 at 60 g/mol, is a third compound with the same empirical formula. This is exactly why combustion analysis reports an empirical formula, not a molecular one.
How do I enter nitrogen, and what does the mass-percent column show?
Set 'Does the compound contain nitrogen?' to Yes and enter the nitrogen mass in grams from a separate Dumas analysis. The calculator then subtracts carbon, hydrogen, and nitrogen from the sample mass so oxygen is still found by difference, and it adds an N row to the results table. The mass-percent column reports each element's share of the total sample mass (the mass of that element divided by the sample mass, times 100), which lets you compare the result against a published elemental analysis, where composition is usually quoted as mass percents.
Provided by AllCalculators.io
Free online calculators for everyday. No registration required.
Estimates for informational purposes only.
Important Disclaimer: Estimates for informational purposes only.
This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.