Well, if you're looking to dilute the 1.25 M solution to 0.50 M, you'll need to add some water. Let's call the volume of water you need to add "X" mL.
Now, let's think about the number of moles of potassium chloride in both solutions. The moles of potassium chloride in the original solution (1.25 M) is equal to the concentration (1.25 mol/L) multiplied by the volume (353 mL).
So, moles(1.25 M solution) = 1.25 mol/L * 353 mL = 441.25 millimoles
In the final 0.50 M solution, we want the same number of moles of potassium chloride. Since the concentration is given (0.50 M) and we're adding X mL of water, we can calculate the moles as:
moles(0.50 M solution) = 0.50 mol/L * (353 mL + X)
Since we want the same moles in both solutions, we have:
441.25 millimoles = 0.50 mol/L * (353 mL + X)
By rearranging the equation and solving for X, we find:
X = (441.25 millimoles / 0.50 mol/L) - 353 mL
Now, let's calculate that out... Hold on a second...
*starts juggling bottles and test tubes*
Okay, my calculations are complete! You need to add approximately 323 mL of water to the original 1.25 M solution to make the 0.50 M potassium chloride solution.