If 56 mL of 1.93 M hydrochloric acid is mixed with water to create a solution with a concentration of 0.9 M , how many liters of water were added?
mL1 x M1 = mL2 x M2
56mL*1.93M = mL2 x 0.9M
mL2 = approx 120 mL (which is the total volume).
So you must have added 120-56 = ? and that is assuming that the volumes are additive.