[Rutherford]

Calculate the concentration of acetic acid in a solution if 50cm³ of that solution were diluted to 1000cm³. From the obtained solution 10cm³ were used and diluted to 100cm³. For the titration of 20cm³ of this solution 12.5cm³ of NaOH, whose 25cm³ react with 20cm³ of an acetic acid solution (c=0.05mol/dm³), were spent.
I get always 5mol/dm³, but in the book answer it is 10mol/dm³. What is the correct answer?

[Borek]

Your answer looks better to me.

[Rutherford]

Thanks very much. There are quite a few wrong answers in my book then.