Posted by Kate on .
I have two questions.
1. How do I predict the formula of ionic compounds when one is a polyatomic ion? Example: Al (+3) and SO4(2).
2. How would I obtain the charge of Hg in the formula HgCl2?

chemistry 
DrBob222,
First you must know the charge on the polyatomic ion. If you know that, as you do in SO4^2, then you make the +charges and the  charges add to zero (since molecules are neutral). Therefore, the formula for aluminum sulfate is Al2(SO4)3.
Here is a web site that gives the polyatomic ions and their charge. http://www.woodrow.org/teachers/chemistry/links/chem1/PolyatomicIonsChart.html
For the second part of the question, go with the charges you know and make all of them equal to zero for the molecule. In HgCl2, we know Cl is 1 each, two Cl ions will be 2; therefore, Hg must bae +2.
Here are some rules for determining the oxidation state of atoms/molecules, ions, etc.
http://dbhs.wvusd.k12.ca.us/webdocs/Redox/RedoxRules.html
Thanks for using Jiskha.