Chemistry, 27.07.2019 11:30 MyLove7817

The ka values for several weak acids are given below. which acid (and its conjugate base) would be the best buffer at ph = 8.0?

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Answer from: zealandcui

Tris and its conjugate base would be the best buffer at pH 8.

Further Explanation:

Buffer solution:

The solutions that oppose any change in their pH on addition of small amounts of acid or base are called buffer solutions. These are formed wither by a weak base and its conjugate acid or a weak acid and its conjugate base.

Henderson-Hasselbalch equation for a buffer solution is as follows:

${\text{pH}} = {\text{p}}{K_{\text{a}}} + {\text{log}}\dfrac{{\left[ {{\text{Base}}} \right]}}{{\left[ {{\text{Acid}}} \right]}}$ …… (1)

A buffer is most effective when the concentration of both acid and its conjugate base is the same. So equation (1) then modifies as follows:

${\text{pH}} = {\text{p}}{K_{\text{a}}}$ …… (2)

Therefore a buffer works best if its pH becomes equal to ${\text{p}}{K_{\text{a}}}$ .

The formula to calculate ${\text{p}}{K_{\text{a}}}$ is as follows:

${\text{p}}{K_{\text{a}}} = - \log {K_{\text{a}}}$ …… (3)

Here, ${K_{\text{a}}}$ is the dissociation constant of acid.

Substitute $\begin{aligned}{\text{p}}{K_{\text{a}}} &=- \log \left( {6.3 \times {{10}^{ - 9}}} \right)a\\&= 8.20\\\end{aligned}$ for ${K_{\text{a}}}$ in equation (3) to calculate ${\text{p}}{K_{\text{a}}}$ of tris.

$\begin{aligned}{\text{p}}{K_{\text{a}}} &= - \log \left( {6.3 \times {{10}^{ - 9}}} \right)a\\&= 8.20\\\end{aligned}$

Substitute $7.9 \times {10^{ - 7}}$ for ${K_{\text{a}}}$ in equation (3) to calculate ${\text{p}}{K_{\text{a}}}$ of MES.

$\begin{aligned}{\text{p}}{K_{\text{a}}} &= - \log \left( {7.9 \times {{10}^{ - 7}}} \right)\\&= 6.10\\\end{aligned}$

Substitute $1.8 \times {10^{ - 5}}$ for ${K_{\text{a}}}$ in equation (3) to calculate ${\text{p}}{K_{\text{a}}}$ of acetic acid.

$\begin{aligned}{\text{p}}{K_{\text{a}}} &= - \log \left( {1.8 \times {{10}^{ - 5}}} \right)\\&= 4.74\\\end{aligned}$

Substitute $1.8 \times {10^{ - 4}}$ for ${K_{\text{a}}}$ in equation (3) to calculate ${\text{p}}{K_{\text{a}}}$ of formic acid.

$\begin{aligned}{\text{p}}{K_{\text{a}}} &= - \log \left( {1.8 \times {{10}^{ - 4}}} \right)\\&= 3.74\\\end{aligned}$

Substitute $3.2 \times {10^{ - 8}}$ for ${K_{\text{a}}}$ in equation (3) to calculate ${\text{p}}{K_{\text{a}}}$ of HEPES.

$\begin{aligned}{\text{p}}{K_{\text{a}}}&= - \log \left( {3.2 \times {{10}^{ - 8}}} \right) \\&= 7.49\\\end{gathered}$

The value of ${\text{p}}{K_{\text{a}}}$ of tris is nearly equal to the required pH. So tris along with its conjugate base would be the best buffer at pH 8.

Learn more:

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Answer details:

Grade: High School

Chapter: Acid, base and salts

Subject: Chemistry

Keywords: tris, acetic acid, formic acid, MES, HEPES, pH, pKa, Henderson, best buffer, 7.49, 8.20, 4.74, 3.74, Ka.

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