Which substance is most likely to form in a precipitation reaction? a. calcium nitrate b. copper(i) nitrate c. iron(i) chloride d. iron(ii) sulfide e. potassium sulfate

The precipitation means the solid form of the compound. Among the given compounds (a) Calcium nitrate Ca(NO₃)₂ is highly soluble and remains in ionic form in the solvent and will not make precipitation. (b) Copper (I) nitrate [Cu(NO₃)] is unstable as copper (II) is always more stable than its mono ionic form thus it can not precipitated in a reaction. (c) Iron (I) chloride (FeCl) is also unstable as Fe only stabilizes in its +3 or +2 form. (d) Iron (II) sulfide (FeS) is precipitated most likely and is an renowned ore of iron. (e) Potassium sulfate (K₂SO₄) is like to remain mostly in its ionic form and has less tend to precipitate.

Thus FeS will be more likely to be precipitated.      

if you mean hydrogen and oxygen then water is the product

the answer is 303.15 or 303