question_answer 1)

What is the maximum concentration of equimolar solution of ferrous sulphate and sodium sulphide so that when mixed in equal volumes, there is no precipitation of iron sulphide? (For iron sulphide, \[{{K}_{sp}}=6.3\times {{10}^{-18}})\]

Answer:

Let solubility of iron sulphide \[FeS\] is \['s'\,mol/L\] \[\therefore \] \[{{K}_{sp}}={{s}^{2}}\] \[s=\sqrt{{{K}_{sp}}}\] \[=\sqrt{6.3\times {{10}^{-18}}}=2.509\times {{10}^{-9}}M\] \[\therefore \]Concentration of \[FeS{{O}_{4}}\] and \[N{{a}_{2}}S\] before mixing equal volume will be\[2\text{ }\times 2.509\text{ }\times {{10}^{-9}}M\], i. e.,\[5.018\text{ }\times \text{ }{{10}^{-9}}\]M. Precipitation will take place when the concentration exceeds this limiting value.