Metanilic acid is an isomer of sulfanilic acid with molecular formula C6H7NO3S and molecular weight 173.18968 g/mol. It is a white powder that is slightly soluble in water. Metanilic acid is an isomer of sulfanilic acid with molecular formula C6H7NO3S and molecular weight 173.18968 g/mol. It is a white powder that is slightly soluble in water.