Question

A 0.4 M buffer solution was prepared with acetic acid and sodium acetate. At pH 5.5, what are the concentrations of acetic acid and acetate ion? The pKa of acetic acid is 4.76. Round the answers to two decimal places. State the units.

Answer 1

Answer: The concentration of acetic acid and sodium acetate (acetate ion) is 0.06 M and 0.34 M respectively

Explanation:

We are given:

Concentration of buffer solution having acetic acid and sodium acetate = 0.4 M

Let the concentration of acetic acid be x M

So, the concentration of sodium acetate will be = (0.4 - x) M

To calculate the concentration of acid for given pH, we use the equation given by Henderson Hasselbalch:

$pH=pK_a+\log(\frac{[salt]}{[acid]})$

$pH=pK_a+\log(\frac{[CH_3COONa]}{[CH_3COOH]})$

where,

$pK_a$ = negative logarithm of acid dissociation constant of acetic acid = 4.76

$[CH_3COONa]=0.4-x$

$[CH_3COOH]=x$

pH = 5.5

Putting values in above equation, we get:

$5.5=4.76+\log(0.4-x}{x})\\\\x=0.062M$

So, concentration of acetic acid = x = 0.06 M

Concentration of sodium acetate = (0.4 - x) = (0.4 - 0.06) = 0.34 M

Hence, the concentration of acetic acid and sodium acetate (acetate ion) is 0.06 M and 0.34 M respectively