Write the chemical formula for the compound formed by each pair of positive and negative ions.

Question

Ba²⁺ and NO₃^-?

Al³⁺ and CO₃^2-?

K⁺ and PO₄^3-?

Answer 1

You must provide the number of each ion that will make the overall ionic compound neutral (0 charge). So look at the charges for Ba²⁺ and NO₃^-. Ba has a +2 charge, so how many –1 charges will give you a sum of 0? A +2 charge requires two –1 charges (two NO₃^–). So the formula would be Ba(NO₃)₂. In the formula, the Ba has a subscript of 1 (not shown; if no subscript it is assumed to be "1"), and the (NO₃) has a subscript of 2.

Al³⁺ and CO₃^2- have a +3 charge and a –2 charge, respectively. You cannot multiply the smaller, –2, by a whole number to equal 3. So now you have to think about what the smallest whole number is that they both go into evenly. Both 3 and 2 go into 6 evenly; so you need two +3 charges (giving +6) to balance three –2 charges (giving –6). So the subscript for Al will be 2, and the subscript for (CO₃) will be 3.

K⁺ and PO₄^3- have a +1 charge and a –3 charge, respectively. How many +1 charges with one –3 charge do you need to equal zero? Another way to say it: Can you multiply the smaller magnitude number, 1, by a whole number to equal the larger magnitude number, 3? Yes, you would multiply 1 by 3 to equal 3, therefore, you need three K⁺¹ for one PO₄^–3. The subscript for K will be 3 for the one PO₄^–3.

K₃PO₄.

Answer 2

Ba(NO₃)₂

Al₂(CO₃)₃

K₃PO₄