How many liters of pure acid should be added to 25 liters of a 60% solution of acid to obtain a 75% acid solution?
You need to add 15 L of pure acid.
The idea behind this problem is that you're adding pure acid to a solution that is 60% v/v acid, so the added acid will change the percent concentration of the starting solution by increasing the volume of solute and the volume of solvent by the same amount.
So, start by calculating how much acid you have in your starting solution
Let's say that
At the same time, the volume of the solution will also incrase by
This means that the target solution's percent concetration by volume will be equal to
Solve this equation for
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.