1. ## Double-Replacement of Compounds

I swear, I'm about to give up. I've been working for HOURS on online homework, and I cannot get these double replacements!

Magnesium Sulfate combines with Barium Hydroxide. Write and balance.

I got Mg_2_(SO_4)_2_+2Ba(OH)_2_----> 2Mg(OH)_2_+Ba_2_(SO_4_)_2_

Of course that was wrong. Why don't you use the charges??? Mg has a +2 charge, and Sulfate has a 2- charge. Why wouldn't I cross them? There has to be something I'm not getting, but I've done so many of these problems and gotten them wrong, I'm just about to give up. I've got the other replacements/combinations down fine.

Thank you so much for hopefully saving my sanity!

Edit: Ok, I just moved onto another problem (which I again got wrong) and Zinc Nitrate is Zn(NO_3_)_2_. Why does that one cross, while the above one doesn't? *sighs*

2. The equation you have written is technically correct. It's just that in stoichiometry of ionic compounds, we often use the empirical formula - it is simpler and doesn't really make a difference as what we're really looking for is relative amounts of each chemical.

The overall equation is
$MgSO_4 + Ba(OH)_2 \rightarrow Mg(OH)_2 + BaSO_4$

Net Ionic Equations:
$Mg^{2+} + 2OH^{-} \rightarrow Mg(OH)_2$
$Ba^{2+} + SO_4^{2-} \rightarrow BaSO_4$

I'm not sure what you meant by crossing, but both the magnesium ion and barium ion have charges of 2+. That means the anions they exchange are also the same charge.