The chemical formula of ammonium chloride is NH₄Cl. It is the salt of a strong acid, HCl and a weak base, NH4OH, so an aqueous solution of ammonium chloride is acidic in nature. When dissolved in water, it gets hydrolysed to some extent to form HCl and NH4OH. HCl being a strong acid is fully ionised and gives a large amount of hydrogen ions whereas NH4OH being a weak base, gets slightly ionised. So, NH4Cl contains more of hydrogen ions than hydroxide ions and hence acidic in nature.