To solve this problem, we need to use the formula for molarity, which is:

Molarity = moles of solute / liters of solution

We can rearrange this formula to solve for the moles of solute:

moles of solute = Molarity x liters of solution

We know the initial molarity of the solution is 0.15 M, and we want to end up with a 0.40 M solution. We also know the initial volume of the solution is 500 mL, or 0.5 L. We can use these values to find the moles of KCl needed to make the new solution:

moles of solute = (0.40 M) x (0.5 L) - (0.15 M) x (0.5 L) moles of solute = 0.125 mol

Now we need to convert the moles of KCl to grams. The molar mass of KCl is 74.55 g/mol. We can use this value to convert moles to grams:

mass of KCl = moles of KCl x molar mass of KCl mass of KCl = 0.125 mol x 74.55 g/mol mass of KCl = 9.32 g

Therefore, we need to add 9.32 grams of KCl to 500 mL of a 0.15 M KCl solution to produce a 0.40 M solution.