Question

1. The removal of H2S gas from filtrate is must before doing analysis of group III and V.
Why?​

Answer 1

Answer:

In qualitative analysis of cations of second group, H2S is passed in presence of HCl. Due to common ion effect, ionisation of H2S decreases and less sulphide ions are obtained. These sulphide ions are sufficient for the precipitation of second group cations in form of their sulphides due to lower value of their solubility product (Ksp). Here, fourth group cations are not precipitated because they required for exceeding their ionic product to their solubility products and highers sulphide ions concentration due to their higher (Ksp)  which is not obtained here due to common ion effect.