Cations formed from transition metals, such as lead, cobalt and iron, usually form insoluble compounds with other ions in solution. However, when they are mixed with nitrate or ammonium ions, no precipitates form. Which of the following provides the best explanation for this observation?
A. Both nitrate and ammonium always form soluble compounds in aqueous solution.
B. Nitrate is always soluble due to its ability to hydrogen bond with water anda ammonium doe snot react with the cations
C. Ammonium is always soluble due to its ability to hydrogen bond with water and nitrate does not react with the cations
D. Both nitrate and ammonium always form insoluble compounds in aqueous solution
B; Answer A may be tempting, especially since it is true. However, think carefully about what is actually happening here. Cations are added to a solution containing nitrate, a negatively charged anion, and ammonium, a positively charged cation. Two cations will never interact with each other. Thus, saying that ammonium compounds are always soluble does not logically apply because no compound would be formed anyway. Answer B is correct. The hydrogen bonding capability of nitrates IS a logical explanation for their solubility and ammonium, being a cation itself, would not react with other cations. You may question whether or not nitrate can form hydrogen bonds because there is no hydrogen directly attached to an F, O or N atom. This does mean that nitrate cannot be a hydrogen bond donor and therefore could not hydrogen bond with itself. However, it can act as a hydrogen bond acceptor. Any F, O or N with a lone pair can act as a hydrogen bond acceptor.