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Question

# What is the maximum concentration of an equimolar solution of ferrous sulphate(FeSO4) and sodium sulphide (Na2S) so that when mixed in equal volumes, there is not precipitation of iron sulphide (FeS)? (For FeS, Ksp=5×10−18)

A
15.5×106 M
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B
4.47×109 M
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C
7.8×1010 M
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D
2.21×109 M
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Solution

## The correct option is B 4.47×10−9 MThe dissociation reactions are given below : FeSO4→Fe2++SO2−4 and Na2S→2Na++S2− Let in the solution the concentration of each FeSO4​ and Na2S is ′s′ M. Since equal volume of FeSO4​ and Na2S are mixed. [FeSO4]=[Na2S]=s2 M. i.e. [F2+]=[S2−]=s2 M. After mixing , FeS is formed. Now, FeS⇌[Fe2+][S2−] Ksp=[Fe2+][S2−]=5×10−18s24=5×10−18s2=20×10−18s=4.47×10−9 M Hence the highest molarity for the solution is ′s′ for which there will be precipitation. ⇒s=4.47×10−9 M

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