So, start by calculating how much acid you have in your starting solution
25
L solution
⋅
60 L acid
100
L solution
=
15 L acid
Let's say that
x
denotes the volume of pure acid that you must add to your starting solution. Since you're dealing with pure acid, the volume of acid will Increase by
x
V
acid
=
15
+
x
At the same time, the volume of the solution will also incrase by
x
V
sol
=
25
+
x
This means that the target solution's percent concetration by volume will be equal to
V
acid
V
soL
⋅
100
=
75
%
(
15
+
x
)
L
(
25
+
x
)
L
⋅
100
=
75
%
Solve this equation for
x
to get
(
15
+
x
)
⋅
100
=
(
25
+
x
)
⋅
75
1500
+
100
x
=
1875
+
75
x
25
x
=
375
⇒
x
=
375
25
=
15 L
So, if you add 15 L of pure acid to 25 L of 60% v/v acid solution, you'll get 40 L of 75% v/v acid solution.
