Question

Suppose you want to prepare a buffer with a pH of 4.59 using formic acid. What ratio of [sodium formate]/[formic acid) do you need to make this buffer? Formic acid has a K, of 1.8x10 4 Enter your answer to three significant figures.

Answer 1

Answer:

7.08

Explanation:

To solve this problem we'll use the Henderson-Hasselbach equation:

• pH = pka + log$\frac{[A^-]}{[HA]}$

Where $\frac{[A^-]}{[HA]}$ is the ratio of [sodium formate]/[formic acid] and pka is equal to -log(Ka), meaning that:

• pka = -log (1.8x10⁻⁴) = 3.74

We input the data:

• 4.59 = 3.74 + log$\frac{[A^-]}{[HA]}$

And solve for $\frac{[A^-]}{[HA]}$:

• 0.85 = log$\frac{[A^-]}{[HA]}$

• $10^{(0.85)}$=$\frac{[A^-]}{[HA]}$

• $\frac{[A^-]}{[HA]}$ = 7.08