Complete and balance the precipitation reactions. Include physical states. Refer to the solubility rules as necessary.

Question

precipitation reaction: K3PO4(aq)+MgCl2(aq)⟶?
I know it would require a double replacement so it would be KCl+Mg(PO)4
But how do I know the coefficients and/or subscripts?

Answer 1

K3PO4(aq) + MgCl2(aq)⟶? KCl+Mg(PO)4 (not correct).
Yes, you're right, it is a double replacement reaction and your products are correct but the formula for magnesium phosphate should be Mg3(PO4)2
K3PO4(aq) + MgCl2(aq)⟶ KCl(aq) + Mg3(PO4)2(s)
Equations of this type are balanced by inspection and everyone does it a different way. I'll go through it and you can ask questions if necessary.
I look and see 3K on the left so place a 3 in front of KCl (you will need to change that later). You see 3 Mg on the right side so place a 3 for Mg on the left. You see a 2 PO4 on the right so place a 2 in front of K3PO4. Yes, that messes up the K that you fixed at the beginning so change that 3 you had for KCl to a 6 and you'll be OK. If I were doing this I have enough experience to know that would happen and I would have started with Mg3(PO4)2 initially. The correct equation is
2K3PO4(aq) + 3MgCl2(aq)⟶ 6KCl(aq) + Mg3(PO4)2(s)
Now you check the final equation to make sure everything is OK.
I see 6K left and right.
I see 2 PO4 left and right.
I see 3 Mg left and right.
I see 6 Cl left and right.
Done. Don't forget that last step of checking. That way you will ALWAYS know if the equation is balanced correctly or not.