100 mL of 1.0 M aqueous NaOH solution was diluted to 1.0 L by adding water. Half of this solution was discarded. A new 100 mL of 0.5 M aqueous NaOH solution was added to the remaining solution. What is the concentration of the final aqueous NaOH solution?

Step-by-step solution

Initial millimoles of NaOH = 100 1.0 = 100 mmol. Volume after dilution = 1.0 L = 1000 mL. Molarity after dilution = 100 1000 = 0.1 M. Half of the solution is discarded, so the remaining volume is 500 mL. Millimoles of NaOH in the remaining solution = 500 0.1 = 50 mmol. Millimoles of NaOH added = 100 0.5 = 50 mmol. Total millimoles of NaOH in the final solution = 50 + 50 = 100 mmol. Total volume of the final solution = 500 + 100 = 600 mL. Final concentration = 100 600 = 1 6 0.1667 M 0.17 M. Answer: 0.17 M