What ratio of [conjugate base]/[acid] would yield the best result for the required buffer solution created in the question (11) above? ENTER YOUR ANSWER AS ONE NUMBER ROUNDED TO TWO DECIMAL PLACES.

Question

PLS ANSWER THIS QUESTION?

Answer 1

First, remember that a buffer consists of EITHER a weak acid and its conjugate base, OR a weak base and its conjugate acid.If the desired pH is under 7, we use a weak acid and its conjugate base. The problem says it wants a pH of 3.50, and 3.50 is less than 7. So we want to use a weak acid and its conjugate base.

The problem gave us a list of substances (acids and bases) we can use to make this buffer. Let's sort through them and figure out which one(s) are a weak acid, and if any the list contains any conjugate bases for the weak acids. Remember that a conjugate base of a weak acid is often written as a salt. Here is the list and the identification of each substance as a weak/strong acid, and its conjugate base of a weak acid in the list:

Substance Name Weak/Strong Acid/Base Conjugate Base

HCOOH Formic acid Weak acid NaHCOO

CH₃COOH Acetic acid Weak acid NaCH₃COO

H₃PO₄ Phosphoric acid Weak acid NaH₂PO₄

So we have three different weak acids and their conjugate bases to choose from. So which should we pick? We should choose the acid with a pKa closest to our desired pH. So we want the acid with a pKa closest to 3.50. Remember that if you know the Ka, the pKa is simply --log (Ka). The pKa for formic acid is 3.75, the pKa for acetic acid is 4.74, and the Ka for phosphoric acid is 2.16. Of these numbers, the closest to 3.50 is obviously 3.75, which is the pKa for formic acid. So we want to use formic acid and its conjugate base.

So we decide to use formic acid (HCOOH) and its conjugate base (NaHCOO). Now we want to find the ratio of [conjugate base]/ [acid]. Remember that we can use the Henderson-Hasselbach equation to figure this out. The Henderson-Hasselbach equation is:

pH = pKa + log( [conjugate base] / [acid] )

We know the desired pH is 3.50. We also know that the pKa of formic acid is 3.75. Remember that the equation for pKa is: pKa = -log (Ka).

So if we put the pKa = 3.75 and the pH = 3.50 into the Henderson-Hasselbach equation:

3.50 = 3.75 + log ( [conjugate base] / [conjugate acid] )

--0.25 = log ( [conjugate base] / [conjugate acid])

10^-0.25 = [conjugate base] / [conjugate acid]

0.5623 = [conjugate base] / [conjugate acid]

So the ratio you want is 0.56 when you round to 2 decimal places.