Let the mass of KCl and NH4Cl in a mixture be a and b respectively.
∴a+b=1.387...(i)
NH4ClΔ−→NH3↑+HCl↑
On heating KCl remains unaffected, whereas NH3Cl is decomposed to give off NH3 and HCl.
The residue left is KCl only which reacts with AgNO3.
Meq. of KCl = Meq. of AgNO3
a74.5×1000=20110
∴a=0.149g (mass of KCl)
By Eq. (i), ∴b=1.387−0.149=1.238g (mass of NH4Cl
Now for mass of Cl2 in mixture
Meq. of Cl2= Meq. of KCl+ Meq. of NH4Cl
Wcl35.5×1000=0.14974.5×1000+1.23853.5×1000
WCl=0.892g
% of chlorine in mixture =0.8921.387×100=64.35%.
So, the answer is 64.