How many grams of O2 are required to precipitate all the iron in 75 mL of 0.090 M Fe2+?

mass O₂ required : 0.054 g

The reaction equation is the chemical formula of reagents and product substances  

A reaction coefficient is a number in the chemical formula of a substance involved in the reaction equation. The reaction coefficient is useful for equalizing reagents and products  

Reaction

4Fe(OH)⁺ (aq) + 4OH⁻ (aq) + O₂ (g)+ 2H₂O (l)⇒ 4Fe(OH)₃ (s)

To precipitate Fe²⁺(in Fe(OH⁺) to Fe³⁺(in Fe(OH)₃)

mol ratio from equation = mol O₂ :mol Fe(OH) = 1 : 4

a). in this case, dh> 0 and sd> 0. if t becomes larger, the term "-tds" will become more negative. at some point, it is possible that -dts will be more negative than the dh is positive, and the reaction will become spontaneous. this is probably the correct answer, but you should look at all of the other possibilities and make sure of that.