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Question

The mass of substance containing 60% NaCl, 37% KCl, that should be weighed out for analysis so that, after the action of 25 mL of 0.1 N AgNO3 solution, excess of Ag+ is back titrated with 5 mL of NH4SCN solution, is__________.
Given that 1 mL of NH4SCN=1.1 mL of AgNO3, write it in multiple of 100 (only two digits).

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Solution

Suppose mass of sample used =a g
Thus, wNaCl=60×a100=0.6a g
wKCl=37×a100=0.37a g
This mixture reacts with AgNO3, the excess of AgNO3 is back titrated by NH4SCN.
Meq. of AgNO3 added to mixture =25×0.1=2.5
The normaliy of NH4SCN can be derived by
Meq. of NH4SCN= Meq. of AgNO3
N×1=0.1×1.1
NNH4SCN=0.11
Meq. of AgNO3 left = Meq. of NH4SCN=5×N
=5×0.11=0.55
Therefore, Meq. of AgNO3 used for mixture
=2.50.55=1.95
or Meq. of KCl+ Meq. of NaCl=1.95
0.37a74.5×1000+0.6a58.5×1000=1.95
a=0.128=013 g
So, answer is 13.

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