Hono

Nitrous acid (molecular formula HNO2) is a weak and monoprotic acid known only in solution, in the gas phase, and in the form of nitrite (NO−2) salts. It was discovered by Carl Wilhelm Scheele, who called it "phlogisticated acid of niter". Nitrous acid is used to make diazonium salts from amines. The resulting diazonium salts are reagents in azo coupling reactions to give azo dyes.