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:

Write the chemical equation responsible for pH of buffer containing and :

Reason for the acidic and basic nature of amino acid.

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.

Answer from: Quest

if the concentration of products is decreased, the equilibrium will shift toward the products). add an inert gas (one that is not involved in the reaction) to the constant-volume reaction mixture: this will increase the total pressure of the system, but will have no effect on the equilibrium condition.

explanation:

Answer from: Quest

the ideal gas law equation is as follows

pv = nrt

where p - pressure

v - volume

n - number of moles

r - universal gas constant

t - temperature

so if the pressure, volume and temperature are already known

we are left with n and r

since r is the universal gas constant that has a known fixed value then r too is known

so we are left with 'n'

once we know temperature volume and pressure

we can find the number of moles of gas present using the ideal gas law equation

this is not my own answer, this is an answer for the same question by the user blahblahmali. i do not claim ownership of this account.

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The ka values for several weak acids are given below. which acid (and its conjugate base) would be t...