To find out how much glacial acetic acid needs to be used to prepare 120 mL of a 25% acetic acid solution, we need to determine the volume of acetic acid in the final solution.
Let's assume x mL of glacial acetic acid is used.
We know that the solution is 25% acetic acid by volume. This means that for every 100 mL of the solution, 25 mL is acetic acid.
So, we can set up the following equation:
x mL of glacial acetic acid / 100 mL of solution = 25 mL of acetic acid / 100 mL of solution
Now we can solve for x:
x / 100 = 25 / 100
Multiplying both sides by 100:
x = (25 / 100) * 100
x = 25 mL
Therefore, 25 mL of glacial acetic acid needs to be used to prepare 120 mL of the desired solution.