To calculate the total moles of KNO3 required to make 1.5 liters of a 2.0 M solution, we can use the formula:

```

Moles of KNO3 = Molarity (M) × Volume of solution (in liters)

```

Substituting the given values into the formula:

```

Moles of KNO3 = 2.0 M × 1.5 L = 3.0 moles

```

Therefore, 3.0 moles of KNO3 must be dissolved in water to make 1.5 liters of a 2.0 M solution.