quantitative estimation

Question 1

Liebig’s Method

Estimation of Carbon and Hydrogen

Answer 1

1. CxHy + ( x+ y/4)O2 gives xCO2 + y/2 H2O
2. percentage of carbon = 12/44 *mass of CO2 *100/mass of organic substance
3. percentage of hydrogen =2/18mass of H2O100/mass of organic substance

Question 2

On heating with CuO, elements other than C and H are also modified as follows :

Answer 2

When organic compound contains nitrogen, the oxides of nitrogen(NO, NO2 etc.) are absorbed by caustic potash. These are removed by the use of bright copper gauge.
4Cu + 2NO2 gives 4CuO + N2
Cu + N2O gives CuO + N2
Nitrogen is not absorbed by KOH solution.

Question 3

Estimation of Nitrogen

(i) Duma’s method

Answer 3

1. it is better than Kjeldahl’s method.
   2.CxHyNz + (2x+y/2) CuO gives x CO2 + y/2 H2O+z/2N2 + (2x+y/2)Cu
   3.Percentage of nitrogen =28volume of N2 at NTP100/22400wt. of organic compound
   mass of nitrogen100/mass of organic substance

Question 4

(ii) Kjeldahl’s method

Answer 4

Organic compound + conc. H2SO4 + small amount of K2SO4 and
CuSO4 gives (NH4)2SO4 +2NaOH gives Na2SO4 +2NH3 +2H20
2NH3 +H2SO4 gives (NH4)2SO4
Percentage of nitrogen = 1.4 N V /mass of organic compound
N = normality of acid
V = volume of acid in mL neutralised by ammonia
It cannot be applied to compounds containing nitrogen directly linked to oxygen or nitrogen such as nitro, nitroso, azo and nitrogen present in ring as in pyridine.

Question 5

Carius Method

Estimation of Halogen

Answer 5

1. Organic compound + Fuming HNO3 + AgNO3 gives AgX

2. Percentage of halogen= Atomic mass of halogen atom* mass of AgX * 100/ mol. mass of AgX * mass of organic compound

Question 6

Estimation of Sulphur

Answer 6

Organic compound + Oxidising agent (Na2O2 or fuming HNO3) gives H2SO4 +BaCl2 gives BaSO4
percentage of sulpher= 32mass of BaSO4100/233*mass of organic compound

Question 7

Estimation of Phosphorus

Answer 7

Organic compound + Fuming nitric acid gives H3PO4+magnesia mixture (MgSO4 + NH4OH + NH4Cl) gives MgNH4PO4 on ignition with magnesium pyrophosphate gives Mg2P2O7
Percentage of phosphorus=62 mass of Mg2P2O7100/222* mass of organic compound

Question 8

Estimation of oxygen

Answer 8

Compound +heat gives O2 +Other gaseous products
2C+O2 gives 2CO5
I2O5+5CO gives I2+5CO22
Percentage of oxygen =32mass of CO2 produced100/88*mass of organic compound taken