[Araconan]

I'm currently doing the following problem:

Calculate the mass of sodium hydroxide that must be added to 1L of 1M HC₂H₃O₂ to double the pH of the solution (assume that the added NaOH does not change the volume of the solution).

What I did:

Calculating the amount of hydrogen/hydronium ions at equilibrium for the solution of acetic acid, yields that the hydrogen ion concentration is 0.0042426407M, with a pH of 2.37.
Doubling the pH would equal a pH of 4.74, with an ion concentration of (1*10^-4.74 = 1.8x10^-5). That means I need to add the amount of hydroxide concentration that will neutralize the amount of hydrogen ions necessary for the final hydrogen ion count to be 1.8x10^-5. 0.0042426407 - 1.8x10^-5 = 0.0042246407 moles of sodium hydroxide required.

The mass is the calculated to be about 0.16897 grams. 

However, the answer is 20 grams, so where did I go wrong?

[fledarmus]

You neglected the large amount of undissociated acetic acid in your solution. What happens to that as you lower the hydronium ion concentration?

[Araconan]

Ahh I see now. Thank you!