Q18 / 54 · JEE Advanced 2020 (Paper 2) · Chemistry

An acidified solution of 0.05 M Zn2+ is saturated with 0.1 M H2S. What is the minimum molar concentration (M) of H+ required to prevent the precipitation of ZnS? Use Ksp(ZnS)=1.25×10-22 and overall dissociation constant of H2S, KNET=K1K2=1×10-21

Correct answer: 0.2

Solution

For ZnS

Ksp(ZnS)=Zn2+×S2-=1.25×10-22

=0.05×S2-=1.25×10-22

S2-=25×10-22

For H2S

H2S0.1 M⇌2H++S2−25×10−22

Ka=H'S2-H2S=1×10-21

=H+2×25×10-220.1=1×10-21

H+2=125

H+=15

H+=0.2 M

 

Hence, the correct answer is 0.2.

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