calculate the pH of 0.25 M formic acid and 0.40 M sodium formate
Use the HH equation.
To calculate the pH of a solution containing a weak acid and its conjugate base, you can use the Henderson-Hasselbalch equation. The Henderson-Hasselbalch equation is given by:
pH = pKa + log([A-]/[HA])
Where:
pH is the negative logarithm of the hydrogen ion concentration.
pKa is the negative logarithm of the acid dissociation constant (Ka) of the weak acid.
[A-] is the concentration of the conjugate base.
[HA] is the concentration of the weak acid.
For this case, formic acid (HCOOH) is a weak acid, and sodium formate (HCOONa) is its conjugate base. The pKa value for formic acid is approximately 3.75.
Step 1: Calculate the ratio [A-]/[HA]:
[A-]/[HA] = [sodium formate] / [formic acid]
[A-]/[HA] = 0.40 M / 0.25 M
[A-]/[HA] = 1.6
Step 2: Substitute the values into the Henderson-Hasselbalch equation:
pH = 3.75 + log(1.6)
pH = 3.75 + 0.2041
pH ≈ 3.95
Therefore, the pH of the solution containing 0.25 M formic acid and 0.40 M sodium formate is approximately 3.95.