A 2.5-L solution with a 70% concentration of acid contains
0.70 (2.5 L) = 1.75 L
of acid.
Suppose you add x L of water, which contributes no additional acid. Then the new volume is (2.5 + x) L, but the amount of acid in the solution stays the same. If the new concentration is 50%, then
1.75 / (2.5 + x) = 0.50
Solve for x :
1.75 = 0.50 (2.5 + x)
1.75 = 1.25 + 0.50x
0.50 = 0.50x
x = 1
