25. A buffer is prepared by mixing picric acid and its conjugate base in a 2:1 ratio. The pH of the solution is measured as 0.079. What is the Ka of picric acid?

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Answer 1 · 2018-02-22T15:32:51

$K_a = 0.417$

Equation for the pH of week acid is:

$pH = pK_a + log [A^-/{HA}]$

where $A^-$ is the concentration of conjugate base.

$HA$ is the concentration of Acid.

$pH = 0.079$ $" "$ $\color(blue){\rightarrow}$ $\text{Given}$

$0.079 = pK_a + log [1/2]$

$0.079 = pK_a -0.3$

$pK_a = 0.079 + 0.3$

$pK_a = 0.38$

Use the equation $\ \ \$ $K_a= 10^{-pK_a}$ $\ \ \$ to calculate the $K_a$ of picric acid.

$K_a = 10^{-0.38}$

$K_a = 0.417$