You are asked to make 500.0 mL of a 4.5M solution of HCl, ?

Question

but the only HCl you have available to dilute is 18.0M. Describe how you would make the desired solution.

Answer 1

4.5 is the 1/4 of 18M

So you take 125mL of 18M , you addd 375 mL of water and you have your solution

since 125/(125+375) = 125/500=1/4

Answer 2

Use the formula

M1V1 = M2V2, where

M1 = molarity of solution you want = 4.5

V1 = volume of solution you want = 0.500L

M2 = molarity of solution you have to use = 18.0

V2 = volume of solution you're going to need = ??

--> (4.5)(0.500) = (18.0)V2

-->0.125L

Thus, all you need to do is take a beaker and pour in 0.125L (or 125 mL) of your 18.0M HCl solution, add it to 375 mL of water (500mL-125mL = 375 and notice number of moles are equal on both sides of the equation) and there you have it.

♠