Ksp of CdS is 8.0x10-27 and that of H2S is 1x 10-22. 1 x 10-14 M CdCl2, solution is precipitated on passing H2S when pH is about

H2S⇌2H++S2−;CdS⇌Cd2++S2− Ksp(CdS)=Cd2+S2− 8×10−27=1×10−14S2− ∴ S2−=8×10−13M  Also, H+2S2−=KspH2S H+28×10−13=1×10−22 H+2=1.25×10−10 ∴ H+=1.11×10−5,pH≈5