Respuesta :
One of the best buffer choice for pH = 8.0 is Tris with Ka value of 6.3 x 10^-9.
To support this answer, we first calculate for the pKa value as the negative logarithm of the Ka value:
pKa = -log Ka
For Tris, which is an abbreviation for 2-Amino-2-hydroxymethyl-propane-1,3 -diol and has a Ka value of 6.3 x 10^-9, the pKa is
pKa = -log Ka
= -log (6.3x10^-9)
= 8.2
We know that buffers work best when pH is equal to pKa:
pKa = 8.2 = pH
Therefore Tris would be a best buffer at pH = 8.0.
To support this answer, we first calculate for the pKa value as the negative logarithm of the Ka value:
pKa = -log Ka
For Tris, which is an abbreviation for 2-Amino-2-hydroxymethyl-propane-1,3 -diol and has a Ka value of 6.3 x 10^-9, the pKa is
pKa = -log Ka
= -log (6.3x10^-9)
= 8.2
We know that buffers work best when pH is equal to pKa:
pKa = 8.2 = pH
Therefore Tris would be a best buffer at pH = 8.0.
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:
[tex]{\text{pH}} = {\text{p}}{K_{\text{a}}} + {\text{log}}\dfrac{{\left[ {{\text{Base}}} \right]}}{{\left[ {{\text{Acid}}} \right]}}[/tex] …… (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:
[tex]{\text{pH}} = {\text{p}}{K_{\text{a}}}[/tex] …… (2)
Therefore a buffer works best if its pH becomes equal to [tex]{\text{p}}{K_{\text{a}}}[/tex].
The formula to calculate [tex]{\text{p}}{K_{\text{a}}}[/tex] is as follows:
[tex]{\text{p}}{K_{\text{a}}} = - \log {K_{\text{a}}}[/tex] …… (3)
Here, [tex]{K_{\text{a}}}[/tex] is the dissociation constant of acid.
Substitute [tex]\begin{aligned}{\text{p}}{K_{\text{a}}} &=- \log \left( {6.3 \times {{10}^{ - 9}}} \right)a\\&= 8.20\\\end{aligned}[/tex] for [tex]{K_{\text{a}}}[/tex] in equation (3) to calculate [tex]{\text{p}}{K_{\text{a}}}[/tex] of tris.
[tex]\begin{aligned}{\text{p}}{K_{\text{a}}} &= - \log \left( {6.3 \times {{10}^{ - 9}}} \right)a\\&= 8.20\\\end{aligned}[/tex]
Substitute [tex]7.9 \times {10^{ - 7}}[/tex] for [tex]{K_{\text{a}}}[/tex] in equation (3) to calculate [tex]{\text{p}}{K_{\text{a}}}[/tex] of MES.
[tex]\begin{aligned}{\text{p}}{K_{\text{a}}} &= - \log \left( {7.9 \times {{10}^{ - 7}}} \right)\\&= 6.10\\\end{aligned}[/tex]
Substitute [tex]1.8 \times {10^{ - 5}}[/tex] for [tex]{K_{\text{a}}}[/tex] in equation (3) to calculate [tex]{\text{p}}{K_{\text{a}}}[/tex] of acetic acid.
[tex]\begin{aligned}{\text{p}}{K_{\text{a}}} &= - \log \left( {1.8 \times {{10}^{ - 5}}} \right)\\&= 4.74\\\end{aligned}[/tex]
Substitute [tex]1.8 \times {10^{ - 4}}[/tex] for [tex]{K_{\text{a}}}[/tex] in equation (3) to calculate [tex]{\text{p}}{K_{\text{a}}}[/tex] of formic acid.
[tex]\begin{aligned}{\text{p}}{K_{\text{a}}} &= - \log \left( {1.8 \times {{10}^{ - 4}}} \right)\\&= 3.74\\\end{aligned}[/tex]
Substitute [tex]3.2 \times {10^{ - 8}}[/tex] for [tex]{K_{\text{a}}}[/tex] in equation (3) to calculate [tex]{\text{p}}{K_{\text{a}}}[/tex] of HEPES.
[tex]\begin{aligned}{\text{p}}{K_{\text{a}}}&= - \log \left( {3.2 \times {{10}^{ - 8}}} \right) \\&= 7.49\\\end{gathered}[/tex]
The value of [tex]{\text{p}}{K_{\text{a}}}[/tex] 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:
Write the chemical equation responsible for pH of buffer containing and : https://brainly.com/question/8851686
Reason for the acidic and basic nature of amino acid. https://brainly.com/question/5050077
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.
