A. $\ce{Cr2O7^2-}$
B. $\ce{H+}$
C. $\ce{OH-}$
D. $\ce{Cr^3+}$
E. $\ce{Ba^2+}$Which of the above is the appropriate ion for each blank in the following series of reactions?
$$\begin{array}{ccc}\ce{OH-} + (5) \xrightarrow{} & \ce{CrO4^2- + H+} & \\ & \ce{v} (6) & \\ & \ce{BaCrO4 (s)} & \ce{<=>[(7)][(8)]} & \ce{Ba^2+ + Cr2O7^2- + H2O}\\ & \text{(yellow precipitate)} & & \ce{v SO2 (g), H+} \\ & & & \ce{BaSO4(s)} + (9) + \ce{H2O} \end{array}$$
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So while studying, I came across this reactions' question, which I believe is called a 'cascade reaction'. I have no idea how to do it. I can figure out a few parts, like ($6$) is obviously $\text{E}$, but how do you do the others?