```
M = m / MW * V
```
Where:
- M is the molarity of the solution (1.20 M)
- m is the mass of KCl we want to find (in grams)
- MW is the molecular weight of KCl (74.55 g/mol)
- V is the volume of the solution (2.00 L)
Rearranging the formula to solve for m, we get:
```
m = M * MW * V
```
Substituting the values, we get:
```
m = 1.20 M * 74.55 g/mol * 2.00 L = 179.04 g
```
Therefore, we need to add 179.04 grams of KCl to 2.00 liters of water to make a 1.20 M solution.
