Use the formula:

```

mass of KCl = (percentage of solution / 100) × mass of water

```

In this case, the percentage of solution is 40% and the mass of water is 100g.

```

mass of KCl = (40 / 100) × 100g = 40g

```

Step 2: Convert grams of KCl to moles.

Use the molecular weight of KCl (74.55 g/mol) to convert grams to moles.

```

moles of KCl = mass of KCl / molecular weight of KCl

```

```

moles of KCl = 40g / 74.55 g/mol = 0.537 moles

```

Step 3: Calculate the mass of water needed.

Subtract the mass of KCl from the total mass of the solution to get the mass of water.

```

mass of water = mass of solution - mass of KCl

```

```

mass of water = 100g - 40g = 60g

```

Step 4: Prepare the solution.

Add 40g of KCl to 60g of water and stir until it is completely dissolved. This will produce 100g of a 40% solution of potassium chloride.