Dilution.

Remembering this simple formula will save your troubles.
M₁L₁ = M₂L₂ 
M stands for MOLARITY.
L stands for LITERS 


Example: 


You have 2.00L of 15.4M HCL and you need 500.0 mL of 3.00M HCl, how much of the original (HCl) concentration do you ned, and how much H20 do you need? Answer in mL


Now plug in the numbers into the formula.


15.4M HCl x L1 = 3.00M HCl x 0.500L


Solve what side has most info (right side in this case)


15.4M HCl x L1 = 1.5 mol HCl


Like math we're trying to find L1


1.5 mol HCl = 100 mL
1.5M


Then find the water needed.


Simple subtract L2 qith L1.


0.500L - 0.100L = 0.400L or 400mL of water is needed


Quick video on how to dilute :)