Intermediate Topic 862 of 1102 Organic Chemistry
Elemental Analysis
Determining percentages of C, H and N to find empirical formulas.
In-depth explanation of Elemental Analysis
Elemental analysis determines the percentage of each element in an organic compound, which is then used to find its empirical formula. Carbon and hydrogen are measured by burning the sample and weighing the carbon dioxide and water produced. Nitrogen is determined by the Dumas or Kjeldahl method, halogens by the Carius method, and sulfur and phosphorus by precipitation as barium sulfate and magnesium pyrophosphate. The mass percentages are converted to moles and divided by the smallest value to give the simplest whole-number ratio of atoms.
Going deeper: Elemental Analysis
Key ideas: Detection by Lassaigne sodium fusion: N gives Prussian blue with FeSO4 and Fe3+, S gives violet with nitroprusside or black PbS, halogens give AgX precipitates (white AgCl, pale yellow AgBr, yellow AgI), with HNO3 first to boil off CN- and S2-. Quantitative: Dumas method gives nitrogen as volume at STP, Kjeldahl gives N as NH3 (not for azo or nitro), carbius gives halogens and sulfur. Percent equals mass of element over mass of compound times 100.
Common mistakes and connections: Students skip nitric acid before halogen testing, giving false results, or use Kjeldahl for nitro compounds. Another error is mis-converting gas volume, using 22.4 L only at STP. Links to empirical formula determination, mole concept and degree of unsaturation.
Visualization of Elemental Analysis
Percentage composition by mass from elemental analysis gives the empirical formula; here ethanol is 52.1 percent C, 13.1 percent H and 34.8 percent O. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Lassaigne's test is a qualitative sodium fusion method that detects nitrogen, sulfur and halogens in an organic compound.
Frequently asked questions about Elemental Analysis
What is Elemental Analysis?
Determining percentages of C, H and N to find empirical formulas.
Is Elemental Analysis a beginner, intermediate or advanced topic?
Elemental Analysis is a intermediate topic in Organic Chemistry. It builds on the vocabulary of beginner topics in this area.
What should I learn before Elemental Analysis?
Study these earlier topics in Organic Chemistry first: Iodoform Reaction, Purification of Organic Compounds, Chromatography.
What comes after Elemental Analysis?
The next topics in Organic Chemistry are Degree of Unsaturation and Acidity of Terminal Alkynes.
Can I practise Elemental Analysis with quizzes?
Yes. The interactive lesson includes a self-check question on Elemental Analysis, and the Organic Chemistry quiz draws on every topic in this area.
Where Elemental Analysis fits in Organic Chemistry
Elemental Analysis is topic 134 of 148 in Organic Chemistry.