Final answer:
The percentage strength of the solution after adding 100 ml of purified water to 600 ml of a 25% solution is 21.43%.
Step-by-step explanation:
When 100 ml of purified water is added to 600 ml of a 25% solution, the percentage strength of the resulting solution can be found by calculating the total volume of the new solution and the volume of the original solute. Initially, there is 150 ml of solute in the 600 ml of 25% solution (because 25% of 600 ml is 150 ml). After adding the purified water, the total solution volume becomes 700 ml (600 ml + 100 ml).
To find the new percentage strength, the volume of the solute (150 ml) is divided by the total volume of the solution (700 ml) and then multiplied by 100%:
Percentage strength = (150 ml / 700 ml) x 100% = 21.43%.