Purification and Analysis of Organic Compounds: Quick Revision

A brief note on Purification and Analysis of Organic Compounds. Read it before a test. Try each question at the end before you open its answer.

Purification

• Sublimation: a solid goes to vapour and back, leaving impurities behind.
• Crystallisation: hot saturated solution, then cooling. The impurities stay in the mother liquor.
• Simple distillation: a large gap in boiling points. Fractional: a small gap, with a column.
• Reduced pressure: for a liquid that decomposes. Steam: for a liquid that does not mix with water.
• Extraction: moves a compound into a solvent that does not mix with water.
• Chromatography: $R_f = \dfrac{d_s}{d_f}$, the distance moved by the substance over the distance moved by the solvent front. It is never more than 1.

Tests for the elements

• Carbon: carbon dioxide turns lime water milky. Hydrogen: water turns white copper sulphate blue.
• Sodium fusion makes CN, S and X ions. Boil with nitric acid before silver nitrate.
• Nitrogen: Prussian blue. Sulphur: black PbS or violet. Both together: blood red.
• Halogens: white (Cl), pale yellow (Br), yellow (I). Phosphorus: yellow with ammonium molybdate.

Quantitative methods

• Liebig: $\%\text{C} = \tfrac{12}{44} \times \tfrac{m_{CO_2}}{m} \times 100$ and $\%\text{H} = \tfrac{2}{18} \times \tfrac{m_{H_2O}}{m} \times 100$.
• Dumas: $\%\text{N} = \tfrac{28 V}{22400\, m} \times 100$ with $V$ at STP in mL.
• Kjeldahl: $\%\text{N} = \tfrac{1.4\, M b V}{m}$. It fails for nitro, azo and ring nitrogen.
• Carius: halogen as AgX, sulphur as BaSO4, phosphorus as $\text{Mg}_2\text{P}_2\text{O}_7$. Oxygen by difference.
• Empirical formula: percentage over atomic mass, then divide by the smallest.

A worked example

A $0.45\ \text{g}$ sample gives $1.10\ \text{g}$ of carbon dioxide and $0.27\ \text{g}$ of water. Find the per cent of carbon and hydrogen.

1. Carbon. $\dfrac{12}{44} \times \dfrac{1.10}{0.45} \times 100 = 66.7$.
2. Hydrogen. $\dfrac{2}{18} \times \dfrac{0.27}{0.45} \times 100 = 6.67$.

Practice with new numbers

A program to try

The program finds the per cent of carbon, hydrogen and sulphur from the mass of each product, with one short function.

Mistakes to avoid

• All the carbon dioxide counted as carbon. Only 12 parts in 44 are carbon.
• Silver nitrate before nitric acid. Cyanide and sulphide give false precipitates.
• Forgetting basicity in Kjeldahl's formula. Sulphuric acid supplies two hydrogen ions.
• Rounding a ratio of 1.5 to 1 or 2. Multiply by 2 instead.
• Measuring Rf from the bottom of the plate. Measure from the starting line.

Check yourself

1. Which method purifies a solid that sublimes, such as camphor?

2. A compound gives a blood red colour with iron(III) chloride. Which two elements does it hold?

3. Why is Kjeldahl's method no good for a nitro compound?

4. A compound has 80 % carbon and 20 % hydrogen. What is its empirical formula?