[kimi85]

The pH of a solution of HCl in water is found to be 2.50.  What volume of water would you add to 1.00 L of this solution to raise the pH to 3.10?

the answer is 3.1 L but I don't know how.

I tried the c1v1 = c2v2 but I didn't got the correct answer.

Thank you.

[jacobcolbert]

I get 2.98 when I do it using c1v1=c2v2 which is kinda 3.1. If yours is different than this, are you making sure to convert the pH to [H₃O⁺]. Or maybe I am missing something. I hate saying anything definitively, I am very insecure.

[kimi85]

I think so.  I think the value 1 x 10^-14 should be used. But I don't know how to put it in an equation..

[jacobcolbert]

I don't see why one would need K_w since HCl is a strong acid. I don't know.

[enahs]

3.1 is a valid answer. And so is 2.98. The question is flawed giving the destination pH as 3.10, two significant digits after the decimal, and the answer to one significant digit after the decimal.

There is a lot of round off error in the question. The question/answer is poor.

To the original poster.
C1V1=C2V2 equates total volume of mol's. This is valid in this case. However, this will be calculating the final volume to obtain the desired pH, that also include what you started with (1L) so you must subtract 1 from the answer this gives you.

It is only valid in this case because there are essentially the same number of mols. This equation is better suited for when your dilution involves adding a concentrated solution to a solvent, it is not so great for when you add more solvent to your original concentrated solution.

[kimi85]

Thank you very much! 
I got 2.98 too.