$$ullet$$ Concentrated sulphuric acid $$(H_2SO_4)$$ is a good drying agent because it has a strong affinity for water $$(H_2O)$$ so it can absorb water very fast. $$ullet$$ As ammonia $$(NH_3)$$ has basic nature, concentrated sulphuric acid can quickly react with it and can form another compound. This is the reason conc. $$H_2SO_4$$ is not used to dry $$NH_3$$.