Question

The pka of a weak acid ha is 4.8. What should be the ratio of [acid]/[salt] of a buffer if ph=5.8 is required

Answer 1

Given,

pKₐ of weak acid, HA = 4.8

pH of the acid, HA = 5.8

The ratio of $\frac{[acid]}{[salt]}$ of a buffer can be calculated from Henderson-Hasselbach equation;

pH = pKₐ + log$\frac{[A⁻]}{[HA]}$

⇒ log$\frac{[A⁻]}{[HA]}$ = pH - pKₐ  

⇒ $\frac{[A⁻]}{[HA]}$ = 10pH-pKa

⇒ $\frac{[A⁻]}{[HA]}$ = 105.8-4.8 = 10¹ = 10

Answer 2

The ratio of $\frac{[acid]}{[salt]}$ is 0.1

Explanation:

To calculate the pH of acidic buffer, we use the equation given by Henderson Hasselbalch:

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

We are given:

$pK_a$ = negative logarithm of acid dissociation constant of weak acid = 4.8

pH = 5.8

Putting values in above equation, we get:

$5.8=4.8+\log(\frac{[salt]}{[acid]})\\\\\frac{[acid]}{[salt]}=0.1$

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