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Discuss the Principle of Estimation of Halogens, Sulphur and Phosphorus Present in an Organic Compound. - Chemistry

Question

Discuss the principle of estimation of halogens, sulphur and phosphorus present in an organic compound.

Solution 1

Estimation of halogens

Halogens are estimated by the Carius method. In this method, a known quantity of organic compound is heated with fuming nitric acid in the presence of silver nitrate, contained in a hard glass tube called the Carius tube, taken in a furnace. Carbon and hydrogen that are present in the compound are oxidized to form COand H2O respectively and the halogen present in the compound is converted to the form of AgX. This AgX is then filtered, washed, dried, and weighed.

Let the mass of organic compound be m g.

Mass of AgX formed = m1 g

1 mol of Agx contains 1 mol of X.

Therefore,

Mass of halogen in m1 g of AgX = (Amatic mass of X xx m_1 g)/"Molecular mass of AgX"

Thus, % of halogen will be = ("Atomatic mass of" X xx m_1 xx 100)/"Molecular mass of AgX x m"

Estimation of Sulphur

In this method, a known quantity of organic compound is heated with either fuming nitric acid or sodium peroxide in a hard glass tube called the Carius tube. Sulphur, present in the compound, is oxidized to form sulphuric acid. On addition of excess of barium chloride to it, the precipitation of barium sulphate takes place. This precipitate is then filtered, washed, dried, and weighed.

Let the mass of organic compound be m g.

Mass of BaSO4 formed = m1 g

1 mol of BaSO4 = 233 g BaSO4 = 32 g of Sulphur

Therefore, m1 g of BaSO4 contains = (32xxm_1)/233g of sulphur

Thus percentage of sulphure = (32 xx m_1 xx 100)/(233xxm)

Estimation of phosphorus

In this method, a known quantity of organic compound is heated with fuming nitric acid. Phosphorus, present in the compound, is oxidized to form phosphoric acid. By adding ammonia and ammonium molybdate to the solution, phosphorus can be precipitated as ammonium phosphomolybdate.

Phosphorus can also be estimated by precipitating it as MgNH4POby adding magnesia mixture, which on ignition yields Mg2P2O7.

Let the mass of organic compound be m g.

Mass of ammonium phosphomolybdate formed = m1 g

Molar mass of ammonium phosphomolybdate = 1877 g

Thus percentage of phosphorus = (31 xx m_1 100)/(1877 xx m)%

If P is estimated as Mg2P2O7,

Then percentage of phosphorus = (62 xx m_1 xx 100)/(222xxm)%

Solution 2

Estimation of halogens: It involves oxidising the organic substance with fuming nitric acid in the presence of silver nitrate. The halogen of the substance is thus converted to silver halide which is separated and weighed:
1Weight of organic compound = W gm
weight of silver halide = x g.

% of halogen = "At wt of halogen xx 100x"/"Mol wt of siliver haldie xx w"

Estimation of sulphur: The organic substance is heated with fuming nitric acid but no silver nitrate is added. The sulphur of the substance is oxidised to sulphuric acid which is then precipitated as barium sulphate by adding excess of barium chloride solution. From the weight of BaSO4 so obtained the percentage of sulphur can be calculated.

% of sulphur = (32 ("At. Weight of S"))/(233("mol weight of BaSO"_4))  xx "weight of  x 100 BaSO"_4/"Weight oforganic compound"

Estimation of phosphorous: The organic substance is heated with fuming nitric acid whereupon phosphorous is oxidised to phosphoric acid. The phosphoric acid is precipitated as ammonium phosphomolybdate, (NH4)3 PO4 .12MOO3, by the addition of ammonia and ammonium molybdate solution which is then separated, dried and weighed.

% of P = (31 xx w_1 xx 100)/(1877 xx w)

where Molar mass of (NH_4)_3 PO_4 . 12MoO_3 = 1877 g

if phosphorus is estimate as Mg_2P_2O_7

% of P = (62xxw_1 xx 100)/(222xxw)%

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NCERT Solution for Chemistry Textbook for Class 11 (2018 (Latest))
Chapter 12: Organic Chemistry - Some Basic Principles and Techniques
Q: 23 | Page no. 363
Solution Discuss the Principle of Estimation of Halogens, Sulphur and Phosphorus Present in an Organic Compound. Concept: Quantitative Analysis of Nitrogen.
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