You have 500 mL of 5 M HCl already made. You need to dilute the solution to 1 M HCl. How much water will you need to add?

You need to add 400mL of water

Explanation:

500mL = 5 M HCI     That means that if you divide both sides by 5

100mL = 1 M HCI         If you need ot get rid of 4 M HCI then you add 400 mL of water because that is what it is equal to

thats how its supposed to be