Write chemical formulas for the compounds formed by the following positive and negative ions: Ba2+ and NO3-, Al3+ and Co3, and K+ and PO43-

Answer 1

It's tough to do this on the internet but follow these instructions. K^+ and PO4^3- will be K3PO4

Write the ions together like this.
KPO4. I like to put parentheses around those that are polyatomic ions like PO4^3-. So
K(PO4)

above the K write its charge of 1 and above the PO4 write its charge 3 (ignore the signs). It's hard to show this because of spacing but I will use dots to space.
1...3
K(PO4)

Now draw a large X. The left downward diagonal starts at the 1 and and ends at the BOTTOM RIGHT of the (PO4). The right diagonal starts at the 3 and ends at the BOTTOM LEFT at the right of the K. Now the numbers at the top follow the diagonals to the bottom and you end up with K3(PO4)1. You don't need to write the ones since they are understood and if the polyatomic ion is just 1 then you can omit the parentheses so you write K3PO4 for potassium phosphate.
Note that with Ba(NO3)2 you can't remove the parentheses.