Which of the following pairs cannot be separated by passing $H_2S$ gas?

The colour of the precipitate produced by adding $NaOH$ solution to $HgCl_2$ is

$A$ solution prepared in conc. $HCl$ sometimes gives white turbidity even in the absence of $1^{st}$ group,when water is added to it. It is due to the presence of

$Fe^{2+}$ ion can be distinguished from $Fe^{3+}$ ion by

$A$ solution of a metal ion,when treated with $KI$,gives a red precipitate which dissolves in excess $KI$ to give a colourless solution. Moreover,the solution of the metal ion,on treatment with a solution of cobalt$(II)$ thiocyanate,gives rise to a deep blue crystalline precipitate. The metal ion is:

In the analysis of Group-$II$ basic radicals,$H_2S$ gas is passed in the presence of dilute $HCl$ because...