Carius method: A known mass of the organic substance is heated strongly with fuming HN0₃. C & H get oxidized to CO₂& H₂O while sulphur is oxidized to sulphuric acid as per the following reaction.
\(C\overset { fum.HN{ O }_{ 3 } }{ \longrightarrow } { CO }_{ 2 }\)
\(2H\overset { fum.HN{ O }_{ 3 } }{ \longrightarrow } { H }_{ 2 }O\)
\(\\ S\longrightarrow { SO }_{ 2 }\overset { O+{ H }_{ 2 }O }{ \longrightarrow } { H }_{ 2 }SO_{ 4 }\)
The resulting solution is treated with excess of BaCI₂ solution H₂SO₄ present in the solution in quantitatively converted into BaSO₄, from the mass of BaSO₄, the mass of sulphur and hence the percentage of sulphur in the compound can be calculated.
Procedure:
A known mass of the organic compound is taken in clean carius tube and added a few mL of fuming HNO₃. The tube is the sealed. It is then placed in an iron tube and heated for about 5 hours. The tube is allowed to cool to temperature and a small hole is made to allow gases produced inside to escape. The carius tube is broken and the content collected in a beaker. Excess of BaCl₂ is added to the beaker H₂SO₄ acid formed as a result of the reaction is converted to BaSO₄. The precipitate of BaSO₄ is filtered, washed, dried and weighed. From the mass of BaSO₄, percentage of S is found.
Mass of the organic compound = w g
Mass of the BaSO₄ formed = x g
233g of BaSO₄ contains 32 g of sulphur
\(\therefore \) x g of BaSO₄ contain \(\left( \frac { 32 }{ 233 } \times \frac { x }{ w } \right) \)
Percentage of sulphur = \(\left( \frac { 32 }{ 233 } \times \frac { x }{ w } \times 100 \right) \)%

Both carbon and hydrogen are estimated by the same method. A known weight of the organic substance is burnt in excess of oxygen and the carbon and hydrogen present in it are oxidized to carbon dioxide and water, respectively.
The weight of carbon dioxide and water thus formed are determined and the amount of carbon and hydrogen in the organic substance is calculated. The apparatus employed for the purpose consists of three units
(1) oxygen supply
(2) combustion tube
(3) absorption apparatus.
(1) Oxygen supply: To remove the moisture from oxygen it is allowed to bubble through sulphuric acid and then passed through aV-tube containing soda lime to remove CO₂. The oxygen gas free from moisture and carbondioxide enters the combustion tube.
(2) Combustion tube: A hard glass tube open at both ends is used for the combustion of the organic substance. It contains (i) an oxidized copper gauze to prevent the backward diffusion of the products of combustion (ii) a porcelain
boat containing a known weight of the organic substance (iii) coarse copper oxide on either side and (iv) an oxidized copper gauze placed towards the end of the combustion tube. The combustion tube is heated by a gas burner.
(3) Absorption Apparatus: The combustion products containing moisture and carbondioxide are then passed through the absorption apparatus which consists of (i) a weighed U'-tube packed with pumice soaked- in cone. H₂SO₄ to absorb water (ii) a set of bulbs containing a strong solution of KOH to absorb CO₂ and finally (iii) a guard tube filled with anhydrous CaCI₂ to prevent the entry of moisture from atmosphere. Procedure: The combustion tube is heated strongly to dry its content. It is then cooled slightly and connected to the absorption apparatus. The other end of the combustion tube is open for a while and the boat containing weighed organic substance is introduced. The tube is again heated strongly till the substance in the boat is burnt away. This takes about 2 hours. Finally, a strong current of oxygen is passed through the combustion tube to sweap away any traces of carbon dioxide or moisture which may be left in it. The If-tube and the potash bulbs are then detached and the increase in weight of each of them is determined.
Calculation:
Weight of the organic substance taken= w g
Increase in weight of H₂O = xg
Increase in weight of CO₂ =yg
18 g of H20 contain 2g of hydrogen
\(\therefore \) x g of H2O contain\(\left( \frac { 2 }{ 18 } \times \frac { x }{ w } \right) \) g of hydrogen
Percentage of hydrogen = \(\left( \frac { 2 }{ 18 } \times \frac { x }{ w } \times 100 \right) \)%
44g of CO₂ contains 12g of carbon
\(\therefore \) y g of CO₂ contain \(\left( \frac { 2 }{ 44 } \times \frac { y }{ w } \right) \) g of carbon 
Percentage of carbon= \(\left( \frac { 2 }{ 44 } \times \frac { y }{ w } \times 100 \right) \)%
Note:
1. If the organic substance under investigation also contain N, it will produce oxides of nitrogen on combustion. A spiral of copper is introduced at the combustion tube, to reduce the oxides of nitrogen to nitrogen which escapes unabsorbed.
2. If the compound contains halogen a well, a spiral of silver is also introduced in the combustion tube. It converts halogen into dilver halide.
3. In case if the substance also contains sulphur, the copper oxide in the combustion tube is replaced by lead chromate. The SO₂ formed during combustion is thus converted to lead sulphate and prevented from passing into the absorption unit.

Tautomerism: It is a special type of functional isomerism in which a single compound exists in two readily inter convertible structures that differ markedly in the relative position of atleast one atomic nucleus, generally hydrogen. The two different structures are known as tautomers. ere are several types of tautomerism and the two important types are  dyad and triad systems.
(a) Dyad system: In this system hydrogen atom oscillates between two directly linked polyvalent atoms.
In this example hydrogen atom oscillates between carbon and nitrogen atom
\(\underset { (hydrogencyanide) }{ H } -C\equiv N\longleftrightarrow \underset { (hydrogenisocyanide }{ H } -N\overset { \rightarrow }{ = } C\)
(b) Triad system: In this 'system hydrogen atom oscillates between three polyvalent atoms. It involves 1,3 migration of hydrogen atom from one polyvalent atom to other within the molecule. e most important type of triad system is keto-enol tautomerism and the two groups of tautomers are ketoform and enol-form. The polyvalent atoms involved are one oxygen and two carbon atoms. Enolisation is a process in which keto-form is converted to enol form. Both tautomeric forms are not equally stable. The less stable form is known as lable form
Example:

(c) Ring chain isomerism: In this type of isomerism, compounds having same molecular formula but differ in terms of bonding of carbon atom to form open chain and cyclic structures for eg:

(i) Fisher projection formula:
This is a method of representing three dimensional structures in two dimension. In this method, the chiral atom(s) lies in the plane of paper. The horizontal substituents are pointing towards the observer and the vertical substituents are away from the observer. Fisher projection formula for tartaric acid is given below.


(ii) Sawhorse projection formula :
Here the bond between two carbon atoms is drawn diagonally and slightly elongated. The lower left hand carbon is considered lying towards the front and the upper right hand carbon towards the back. The Fischer projection inadequately portrays the spatial relationship between ligands attached to adjacent atoms. The sawhorse projection attempts to clarify the relative location of the groups.

(iii) Newman projection- formula:
In this method the molecules are viewed from the front along the carbon-carbon bond axis. The two carbon atom forming the bond is represented by two circles. One behind the other so that only the front carbon is seen. The front carbon atom is shown by a point where as the carbon lying further from the eye is represented by the origin of the circle. Therefore, the C-H bonds of the front carbon are depicted from the circle while C-H bonds of the back carbon are drawn from the circumference of the circle with an angle of 1200 to each other.

(i) They are covalent compounds of carbon and generally insoluble in water and readily soluble in organic solvent such as benzene, toluene, ether, chloroform, etc ...
(ii) Many of the organic compounds are inflammable  (except CCI₄).They possess low boiling and melting points due to their covalent nature.
(iii) Organic compounds are characterised by functional groups. A functional group is an atom or a specific combination of bonded atoms that react in a characteristic way, irrespective of the organic molecule in which it is present. In
almost all the cases, the reaction of an organic compound takes place at the functional group. They exhibit isomerism which is a unique phenomenon.
(iv) Homologous series: A series of organic compounds each containing a characteristic functional group and the successive members differ from each other in molecular formula by a CH₂ group is called homologous series.
Alkanes: Methane (CH₄) Ethane (C₂H₆)  Propane (C₃H₈) etc.