Unit 11: Separating & Analyzing Organic Compounds
Mastering the essential laboratory techniques for isolating, purifying, and identifying organic substances.
11.28 Separation Techniques
After a chemical reaction, the desired product is often mixed with unreacted starting materials, solvents, and by-products. Several techniques are used to separate these components based on their physical properties.
- Filtration: Used to separate an insoluble solid from a liquid. The mixture is poured through filter paper, which allows the liquid (filtrate) to pass through but traps the solid (residue).
- Distillation: Used to separate two or more miscible liquids with different boiling points. The liquid with the lower boiling point evaporates first, and its vapour is then condensed and collected separately. It is also used to separate a liquid solvent from a dissolved solid.
- Crystallisation: Used to separate a dissolved solid from a solution. The solvent is partially evaporated to create a saturated solution, which is then cooled. As the solubility of the solid decreases at lower temperatures, it forms pure crystals that can be filtered off.
- Chromatography: A powerful technique used to separate small amounts of substances in a mixture. It works by passing the mixture (the mobile phase) over a stationary material (the stationary phase). Components that are more soluble in the mobile phase or adsorb less strongly to the stationary phase travel faster, causing separation.
Solved Examples:
- Which technique would you use to separate sand from a salt water
solution?
Solution: Filtration, to remove the insoluble sand. - How can you separate pure ethanol from a fermented mixture of ethanol and
water?
Solution: Fractional distillation, because ethanol and water have different boiling points (78°C and 100°C). - A student prepares copper(II) sulphate by reacting excess insoluble
copper(II) oxide with sulphuric acid. Which technique is used to remove the
excess copper(II) oxide?
Solution: Filtration. - What is the principle behind
chromatography?
Solution: Separation based on the differential distribution of components between a stationary phase and a mobile phase. - How would you obtain pure, solid sodium chloride from a salt water
solution?
Solution: Crystallisation. - What is the liquid collected after distillation
called?
Solution: The distillate. - What is the solid left on the filter paper after filtration
called?
Solution: The residue. - Which technique is best for separating the different coloured pigments in a
sample of ink?
Solution: Chromatography. - Why is a solution often cooled to induce
crystallisation?
Solution: Because the solubility of most solids decreases as the temperature decreases. - What does "miscible" mean?
Solution: It means that two liquids can mix together completely to form a single solution (e.g., ethanol and water).
11.29 Purification Techniques
After a product has been separated, it may still contain impurities. Purification techniques are used to obtain a pure sample.
Drying
Small amounts of a solvent (like water) can be removed from a solid product by drying. This can be done by pressing the solid between filter papers, placing it in a low-temperature oven, or leaving it in a desiccator (a sealed container with a chemical drying agent like anhydrous calcium chloride).
Recrystallisation
Recrystallisation is the most common method for purifying an impure solid. It relies on the principle that the desired compound is highly soluble in a hot solvent but much less soluble in the same solvent when it is cold, while the impurities have different solubility characteristics.
- Dissolve: Dissolve the impure solid in the minimum amount of a suitable hot solvent.
- Hot Filter: Quickly filter the hot solution to remove any insoluble impurities.
- Cool: Allow the hot filtrate to cool slowly. As it cools, the desired compound becomes less soluble and forms pure crystals. The soluble impurities remain dissolved in the solvent.
- Cold Filter: Filter the cold mixture to collect the pure crystals.
- Wash and Dry: Wash the crystals with a small amount of cold solvent to remove any remaining impurities, then dry them.
Solved Examples:
- What is the purpose of
recrystallisation?
Solution: To purify an impure solid compound. - What is a key property of a good recrystallisation
solvent?
Solution: The desired compound should be very soluble in it when hot but sparingly soluble when cold. - Why is the minimum amount of hot solvent used in
recrystallisation?
Solution: To ensure the solution is saturated upon cooling, which maximises the yield of crystals recovered. - What is a desiccator used for?
Solution: For drying a solid sample in a moisture-free environment. - During recrystallisation, why is the solution filtered while
hot?
Solution: To remove any insoluble impurities before the desired product crystallises. - Why are the collected crystals washed with cold solvent, not hot
solvent?
Solution: To wash away soluble impurities without redissolving a significant amount of the pure product crystals. - What is the name of the chemical drying agent often found in a
desiccator?
Solution: Anhydrous calcium chloride or silica gel. - What happens to the soluble impurities during the cooling step of
recrystallisation?
Solution: They remain dissolved in the cold solvent (the mother liquor). - How can you check the purity of a recrystallised
solid?
Solution: By measuring its melting point. A pure solid will have a sharp melting point that matches the known literature value. - What is the first step in purifying a crude sample of
aspirin?
Solution: Recrystallisation.
11.30 Analytical Techniques (Molecular Mass/Formula)
Once a pure compound is obtained, analytical techniques are used to determine its identity and structure.
Determining Molecular Formula:
A common method involves two steps:
- Combustion Analysis: A known mass of the pure organic compound is
burned completely in excess oxygen. The mass of the carbon dioxide and water produced is
measured. From this data, the mass of C and H in the original sample can be calculated,
and the empirical formula can be determined.
- Mass of C = (Mass of $CO_2$) × (12.0 / 44.0)
- Mass of H = (Mass of $H_2O$) × (2.0 / 18.0)
- Determining Molar Mass: The molar mass of the compound is found using a technique like mass spectrometry.
By comparing the mass of the empirical formula with the experimentally determined molar mass, the molecular formula can be deduced.
Solved Examples:
- What is the purpose of combustion
analysis?
Solution: To determine the empirical formula of an organic compound. - A 1.00g sample of a hydrocarbon is burned, producing 3.00g of $CO_2$. What
is the mass of carbon in the sample?
Solution: Mass of C = 3.00g × (12.0 / 44.0) = 0.818 g. - What is the first step in determining a molecular formula from experimental
data?
Solution: Find the empirical formula from the percentage composition or combustion data. - A compound has an empirical formula of $CH_2$ and a molar mass of 56 g/mol.
What is its molecular formula?
Solution: Mass of empirical unit ($CH_2$) = 14 g/mol. Multiplier = 56 / 14 = 4. Molecular formula is $(CH_2)_4$ = $C_4H_8$. - What are the products of the complete combustion of a compound containing C,
H, and O?
Solution: Carbon dioxide ($CO_2$) and water ($H_2O$). - How is the mass of oxygen in a compound determined from combustion
analysis?
Solution: It is found by difference: Mass of O = (Total mass of sample) - (Mass of C) - (Mass of H). - What does a mass spectrometer
measure?
Solution: It measures the mass-to-charge ratio of ions, which allows for the determination of the accurate molar mass of a compound. - Combustion of 0.46g of a compound produced 0.88g of $CO_2$ and 0.54g of
$H_2O$. What is its empirical formula?
Solution: Mass C = 0.88 × (12/44) = 0.24g. Moles C = 0.24/12 = 0.02. Mass H = 0.54 × (2/18) = 0.06g. Moles H = 0.06/1 = 0.06. Mass O = 0.46 - 0.24 - 0.06 = 0.16g. Moles O = 0.16/16 = 0.01. Ratio C:H:O = 0.02:0.06:0.01 = 2:6:1. Empirical formula is $C_2H_6O$. - What is the difference between an empirical and a molecular
formula?
Solution: The empirical formula is the simplest whole-number ratio of atoms, while the molecular formula is the actual number of atoms in one molecule. - What information, in addition to the empirical formula, is needed to find
the molecular formula?
Solution: The molar mass (or relative molecular mass) of the compound.
🧠Knowledge Check (20 Questions)
Solution: Different solubilities in a solvent at hot and cold temperatures.