How do you balance #Zn(NO_3)_2 + AlI_3 -> ZnI_2 + Al(NO_3)3#?

Here's a trick, try substituting in variables for the different ions if you get stuck on balancing an equation. It cuts down on clutter when you are working on balancing and makes it easier to solve.

Since this is a double replacement reaction, you only need to balance the numbers of the four ions - this is easier than thinking you need to balance both the N and O separately.

Zn = A
#NO_3# = B
Al = C
I = D

Re-write the equation using the variables and the subscripts from the skeleton (unbalanced equation).

#AB_2 + CD_3 -> AD_2 + CB_3#

Balance all the variables using common multiples
#3AB_2 + 2CD_3 -> 3AD_2 + 2CB_3#

Now re-write the original equation using these coefficients.

#3Zn(NO_3)_2 + 2AlI_3 -> 3ZnI_2 + 2Al(NO_3)_3#

Check to see that the ions all balance correctly, make sure that you can't reduce the ratios and then you are done!

Here is a similar problem:

Hope this helps!