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Page # 99
STOICHIOMETRY - 1
(c)
Kjeldahl’s method : (for nitrogen)
(w) O.C. + H2SO4
NaOH
(NH4 ) 2SO 4
NH3 + H2SO 4
(molarity M and volume (V1) consumed given)
MV1 2 14
100
w
where M = molarity of H 2SO4 .
% of N =
•
(d)
Some N containing compounds do not give the above set of reaction as in Kjeldahl’s method.
Sulphur :
(w) O.C. + HNO 3
% of S =
H2SO 4 + BaCl2
w1
233
×1×
(w1 ) BaSO4
32
× 100
w
where w1 = wt. of Ba SO 4 , w = wt. of organic compound
(e)
Phosphorus :
O.C+ HNO 3
H 3PO 4 + [NH3 + magnesia mixture ammonium molybdate]
MgNH4 PO4
Mg2 P2O7
w 1 2 31
100
222
w
Carius method : (Halogens)
% of P =
(f)
O.C. + HNO 3 + AgNO3
AgX
If X is Cl then colour = white
If X is Br then colour = dull yellow
If X is I then colour = bright yellow
•
Flourine can’t be estimated by this
w1
(M. weight of AgX )
% of X
1 ( At. wt of X )
100
w
Ex.
0.607 g of a silver salt of a tribasic organic acid was quantitatively reduced to 0.370 g of pure
silver. Calculate the molecular weight of the acid (Ag = 108)
Sol.
Suppose the tribasic acid is H 3A.
H3 A
acid
Ag3 A
Ag
salt
0.607 g 0.37 g
Since Ag atoms are conserved, applying POAC for Ag atoms,
moles of Ag atoms in Ag 3A = moles of Ag atoms in the prduct
3 × moles of Ag3 A = moles of Ag in the product
3
0.607
mol. wt. of Ag3 A
0.37
108
(Ag = 108)
mol. wt. of Ag3 A = 531.
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