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```Page # 95
STOICHIOMETRY - 1
Shortcut for % determination if average atomic weight is given for X having isotopes X A & XB.
% of X A
Average atomic wei ght â wt of X B
difference in weight of X A & X B
100
Try working out of such a shortcut for X A, X B, XC
EMPIRICAL FORMULA, MOLECULAR FORMULA
Empirical formula : Formula depicting constituent atom in their simplest ratio.
Molecular formula : Formula depicting actual number of atoms in one molecule of the compound.
Relation between the two : Molecular formula = Empirical formula Ã n
n
Molecular mass
Empirical Formula mass
Check out the importance of each step involved in calculations of empirical formula.
Ex.
Sol.
A molecule of a compound have 13 carbon atoms, 12 hydrogen atom, 3 oxygen atoms and
3.02 Ã 10 â23 gm of other element. Find the molecular wt. of compound.
12
1
16
wt. of the 1 molecule of a compound = 13 Ã N + 12 Ã N + 3 Ã N + 3.02 Ã 10 â23
A
A
A
156 12 48 3. 02 10 â 23 NA
= 234.18 / N A = 234 amu. Ans.
NA
â¢
Density :
(a) Absolute density
(b) Relative density
Absolute density =
Relative density =
Mass
volume
density of subs tan ce
density of s tan dard subs tan ce
density of subs tan ce
Specific gravity = density of H O at 4 C
2
Vapour density : It is defined only for gas.
It is a density of gas with respect to H 2 gas at same temp & press
PMgas / RT
Mgas
d gas
M
V.D = dH = PM / RT = M
=
H
H
2
2
2
2
V.D =
M
2
Molecular wt of gas
V.D = Molecular wt of H gas
2
â¢
density of Cl2 gas with respect to O2 gas
Molecularwt of Cl2 gas
= Molecular wt of O gas
2
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```
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