A solution of formic acid 0.20 M has a pH of 5.0. What is its Ka value?
a) 5.0 x 10-10
b) 1.0 x 10-5
c) 5.0 x 10-5
d) 25
To determine the Ka value for formic acid (HCOOH) based on its pH, we need to use the Henderson-Hasselbalch equation. The Henderson-Hasselbalch equation relates the pH of a solution to its acid dissociation constant, Ka, and the concentrations of the acid and its conjugate base.
The Henderson-Hasselbalch equation is given as:
pH = pKa + log([A-]/[HA])
Where pH is the given pH of the solution, pKa is the negative logarithm of the acid dissociation constant, [A-] is the concentration of the conjugate base (in this case, formate ion, HCOO-), and [HA] is the concentration of the acid (formic acid, HCOOH).
By rearranging the Henderson-Hasselbalch equation, we can solve for the pKa value:
pKa = pH - log([A-]/[HA])
Given that the pH of the solution is 5.0, we can substitute the values into the equation and solve for pKa:
pKa = 5.0 - log([A-]/[HA])
However, in this case, the concentration of the acid [HA] is not given. Therefore, we need additional information to determine the Ka value.
In this question, we cannot calculate the Ka value without knowing the concentration of formic acid [HA]. The options provided do not provide the necessary information to calculate the Ka value.
Convert pH of 5.0 to H^+. You should get 1E-5.
Set up ionization as we did HBz.
Set up ICE chart as in HBz but here you have H^+ given.
Substitute into Ka expression and solve for Ka.