Which combination will produce a precipitate? agno3 (aq) and ca(c2h3o2)2 (aq) naoh (aq) and hcl (aq) nacl (aq) and hc2h3o2 (aq) nh4oh (aq) and hcl (aq) fe(no3)2 (aq) and koh (aq)?

The combination that will produce a precipitate is Fe(NO₃)₂(aq) and KOH(aq). The balanced chemical equation shows the production of iron(II) oxide, Fe(OH)₂(s) and potassium nitrate, KNO₃.

Fe(NO₃)₂(aq) + 2KOH(aq) → Fe(OH)₂(s) + 2KNO₃(aq)

Fe(OH)₂ is insoluble in water. It is naturally a white solid but in the presence of even a small amount of oxygen, it turns into a green color. That is why, it is also referred to as "green rust". The other product KNO₃ is an ionic salt that forms an aqueous solution in water. 

Other combinations among the choices do not produce a solid. This is based on the solubility rules wherein it is stated that:

1. All compounds of C₂H₃O₂, Na⁺, NH₄⁺ along with other alkali metals are soluble in water.
2. All compounds with Cl are soluble except for Pb, Hg, and Ag.

What are the following units