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?
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/Redox-Rules.html
Thanks for using Jiskha.
Sure, I can help you with both questions!
1. To predict the formula of an ionic compound when one is a polyatomic ion, you need to first determine the charges of the individual ions. In your example, Al has a charge of +3 and SO4 has a charge of -2. The next step is to balance these charges to create a neutral compound. To do this, you need to find the least common multiple between the charges. In this case, the least common multiple of 3 and 2 is 6.
To balance the charges, you can multiply the charge of Al by 2 and the charge of SO4 by 3. This gives you Al2(+3) and SO4(-6). The charges now balance out, but keep in mind that you need to write the formula using the correct subscripts.
Since the charge of Al is +3, you would need two Al ions to balance the charge of six (+3 x 2 = +6). Therefore, the formula becomes Al2(SO4)3.
2. To obtain the charge of Hg in the formula HgCl2, you need to consider the overall charge of the compound and the charge of the other ion involved. In this case, Cl is a halide ion and has a charge of -1. Since the overall charge of HgCl2 should be neutral, the sum of the charges must equal zero.
Since you have two Cl ions, each with a charge of -1, the total charge from the Cl ions is -2 (-1 x 2 = -2). Therefore, the charge of Hg should be +2 so that when added to the -2 charge from the Cl ions, it becomes neutral. Hence, the charge of Hg in HgCl2 is +2.
To summarize, to predict the formula of an ionic compound with a polyatomic ion, you balance the charges by finding the least common multiple. And to find the charge of an element in a compound, you consider the overall charge of the compound and the charges of the other ions involved.