Consider the chemical reaction represented by the equation below: 3 MgCl2 + 2 Al = 3 Mg + 2 AlCl3
If 12 moles of magnesium chloride react with enough aluminum, how many moles of aluminum chloride are produced?

3 MgCl2 + 2 Al = 3 Mg + 2 AlCl3

Since we know that we have 12 moles of MgCl2 and enough Al, we don't need to figure out the exact amount of Al.

3 MgCl2 / 2 AlCl3  =  12 mol MgCl2 / x mol AlCl2

cross-multiply

3 MgCl2(x mol AlCl2)  =  12 mol MgCl2(2 AlCl3)

3x = 24

x=8

8 moles AlCl3

Alternatively, we know that we have 12 moles of MgCl2, and a coefficient of 3. This indicates a multiplier of 4 for the entire equation, meaning 2 AlCl3 multiplied by (4) = 8 moles