Question

The Ka values for several weak acids are given below. Which acid (and its conjugate base) would be the best buffer at pH 3.7?a. MES: Ka 7.9 x 10b. HEPES; Ka 3.2 x 103c. Tris; Ka 6.3 x 109d. Formic acid: K 1.8 x 10
e. Acetic acid: K 1.8 x 10

Answer 1

Answer:

Formic acid and Acetic acid is the best buffer at pH 3.7.

Explanation:

Given that,

The Ka values for several weak acids are given,

$K_{a}\ of\ MES=7.9\times10^{-7}$

$K_{a}\ of\ HEPES = 3.2\times10^{-3}$

$K_{a}\ of\ Tris=6.3\times10^{-9}$

$K_{a}\ of\ formic\ acid = 1.8\times10^{-4}$

$K_{a}\ of\ Acetic\ acid = 1.8\times10^{-5}$

We need to calculate the pH of the weak acids with their $pK_{a}$ values

Using formula of $pK_{a}$

For MES,

$pK_{a}=-log K_{a}$

Put the value into the formula

$pK_{a}=-log(7.9\times10^{-7})$

$pK_{a}=7.0-log7.9$

$pK_{a}=6.1$

pH range for best buffer,

$pH=pK_{a}\pm 1$

Put the value into the formula

$pH=6.1\pm 1$

$pH=7.1, 5.1$

The pH value of the solution between 7.1 to 5.1.

This is not best buffer.

For HEPES,

$pK_{a}=-log K_{a}$

Put the value into the formula

$pK_{a}=-log(3.2\times10^{-3})$

$pK_{a}=3.0-log3.2$

$pK_{a}=2.5$

pH range for best buffer,

$pH=pK_{a}\pm 1$

Put the value into the formula

$pH=2.5\pm 1$

$pH=3.5, 1.5$

The pH value of the solution between 3.5 to 1.5.

This is not best buffer.

For Tris,

$pK_{a}=-log K_{a}$

Put the value into the formula

$pK_{a}=-log(6.3\times10^{-9})$

$pK_{a}=9.0-log6.3$

$pK_{a}=8.2$

pH range for best buffer,

$pH=pK_{a}\pm 1$

Put the value into the formula

$pH=8.2\pm 1$

$pH=9.2, 7.2$

The pH value of the solution between 9.2 to 7.2.

This is not best buffer.

For formic acid,

$pK_{a}=-log K_{a}$

Put the value into the formula

$pK_{a}=-log(1.8\times10^{-4})$

$pK_{a}=4.0-log1.8$

$pK_{a}=3.7$

pH range for best buffer,

$pH=pK_{a}\pm 1$

Put the value into the formula

$pH=3.7\pm 1$

$pH=4.7, 2.7$

The pH value of the solution between 4.7 to 2.7.

This is best buffer.

For acetic acid,

$pK_{a}=-log K_{a}$

Put the value into the formula

$pK_{a}=-log(1.8\times10^{-5})$

$pK_{a}=5.0-log1.8$

$pK_{a}=4.7$

pH range for best buffer,

$pH=pK_{a}\pm 1$

Put the value into the formula

$pH=4.7\pm 1$

$pH=5.7, 3.7$

The pH value of the solution between 5.7 to 3.7.

This is  best buffer

Hence, Formic acid and Acetic acid is the best buffer at pH 3.7.