Acid Base Titration: Finding pH

[nobb]

Hi.
I am having trouble with acid-base titration excercises. 0.02 L butanoic acid @ 0.1 M is titrated with 0.01 L NaOH solution @ 0.1 M. The Ka of butanoic acid is 1.54X10^-5. Find the pH. The answer is 4.81. I know how to find the pH when no NaOH is added, but I really don't see how the Ka fits in when adding a base. An explanation would really be appreciated. Thanks.

 

[Borek]

http://www.chembuddy.com/?left=pH-calculation&right=titration-curves-calculation

First you have a weak acid solution, then it is a buffer so you should use Henderson-Hasselbalch equation and finally you have conjugate base solution with pH defined by hydrolyzis.

 

[Blueshift5]

Hello,

Thank you for your question. Acid-base titration is a common laboratory technique used to determine the concentration of an acid or base in a solution. In this case, you are trying to find the pH of a solution after a titration has occurred.

To understand how the Ka fits into this scenario, we need to first understand what Ka represents. Ka is the acid dissociation constant and it tells us about the strength of an acid. The higher the Ka value, the stronger the acid. In this case, the Ka of butanoic acid is 1.54x10^-5, which means it is a weak acid.

When you add a strong base, such as NaOH, to a weak acid solution, a reaction occurs where the base neutralizes the acid. This is known as a neutralization reaction. The products of this reaction are a salt and water. In your case, the salt formed is sodium butanoate.

The Ka of an acid is related to its pKa, which is the negative logarithm of the Ka value. The pKa of butanoic acid is 4.81, which is the same as the pH of the solution after the titration has occurred. This is because at the equivalence point, where the moles of acid and base are equal, the solution is neutral and the pH is equal to the pKa.

To calculate the pH of the solution after the titration, you can use the Henderson-Hasselbalch equation, which is pH = pKa + log([salt]/[acid]). In this case, the concentration of the salt (sodium butanoate) is equal to the concentration of the base (NaOH) added during the titration, which is 0.01 L x 0.1 M = 0.001 mol. The concentration of the acid (butanoic acid) can be calculated using the formula M1V1 = M2V2, where M1 is the initial concentration of the acid (0.1 M), V1 is the initial volume (0.02 L), M2 is the final concentration (unknown), and V2 is the final volume (0.03 L). Solving for M2, we get a concentration of 0.0067 M.

Substituting these values into the Henderson-Hasselbalch equation, we get pH = 4.81 + log(0.001/0.0067) =