Consider the sulphides \( \mathrm{HgS} \), \( \mathrm{PbS} \), \( \mathrm{CuS} \), \( \mathrm{Sb_2S_3} \), \( \mathrm{As_2S_3} \) and \( \mathrm{CdS} \). Number of these sulphides soluble in \( 50\% \) \( \mathrm{HNO_3} \) is _________ .
