Question

When h2so4 react with unknown salt then a brown colour gas evolved?

Answer 1

any metal salt of nitrate -ve radicle that has electro negativity difference more than h+  so that it can displace the elements from the sulphuric acid aq.

the gas evolved is N02 nitrogen dioxide

example

h2so4 + pb(N03)2 ------> PBSO4 + NO2  + H20

UNBALANCED