What is the pH of 100.00 mL solution that contains 10.00 mmol of formic acid and 20.00 mmol of sodium formate.

Answer 1

We first will need the Ka or pKa for formic acid (HCOOH). I found the pKa to be 3.75.

Next, since we have a weak acid (formic acid) and the salt of that acid (sodium formate), we have a BUFFER.

We can thus use the Henderson Hasselbalch equation: pH = pKa + log [salt]/[acid]

[salt] = 20.00 mmol/0.1 L = 200 mM

[acid] = 10.00 mmol/0.1 L = 100 mM

pH = 3.75 + log (200/100)

pH = 3.75 + 0.30

pH = 4.05