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

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

Step-by-step explanation:

Given that,

The Ka values for several weak acids are given,

`K_(a)\ of\ MES=7.9*10^(-7)`

`K_(a)\ of\ HEPES = 3.2*10^(-3)`

`K_(a)\ of\ Tris=6.3*10^(-9)`

`K_(a)\ of\ formic\ acid = 1.8*10^(-4)`

`K_(a)\ of\ Acetic\ acid = 1.8*10^(-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*10^(-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*10^(-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*10^(-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*10^(-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*10^(-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.