In the lab a 3.3mol per liter solution was prepared by adding H2O to 140mL of a 4.3mol Per Liter solution of HCL. How much water was added?
You really don't know because volumes are not additive; however, you can get a very close estimate.
Use c1v1 = c2v2
c = concn
v = volume
4.3M x 140 mL = 3.3M x mL
Solve for mL (which is the total volume). The calculation so far is accurate. To find how much H2O was added you use total volume - 140 = added H2O. That is the estimate but with solutions this dilute it won't miss the mark far.