TOLC-med 2020 buffer system

The pH of a solution containing 0,4M of formic acid and 1,0M sodium formate is 4,15.
What will me the pH of the solution if it is diluted 10 times?

Answer = 4,15

Hi everyone,

here is a worked solution i found online:
A characteristic of a buffer system is that it does not depend on dilution, in other words the pH value guaranteed by a buffer system depends only on the quantity of acid and conjugate base present and not on the value of water. The dilution of our buffer solution therefore does not alter the pH of our solution.

I understand what is said, my problem is how do we know formic acid and sodium formate will form a buffer?


Formic acid (HCOOH) is a weak acid, and sodium formate (HCOONa) is the salt of the conjugate base (formate ion, HCOO-) of that weak acid. When dissolved in water, sodium formate dissociates completely into sodium ions (Na+) and formate ions (HCOO-).

In a buffer solution, the weak acid (formic acid) can donate a proton (H+) to the solution when a base is added, and the conjugate base (formate ion) can accept a proton when an acid is added, maintaining the pH of the solution relatively constant.

The fact that you have a weak acid (formic acid) and its conjugate base (from sodium formate) in the same solution makes this a buffer solution. The ability of the buffer solution to maintain a relatively constant pH, even when diluted, is due to the presence of both the weak acid and its conjugate base in the solution. This is known as the buffer capacity.

In this question, you must know how to convert the name into the chemical compound and know what formic acid and sodium formate look like, it is required on the test di medicine, but it never appeared on the IMAT.

Even if you didn’t know how to convert them and their actual structure, the suffix “ic” and “ate” should let you know that they are most likely very close to each other, usually with a single atom that differs (meaning a buffer).

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