Calculate the pH

Question

calculate the pH of (a) a solution that is 0.250 M in sodium formate (HCOONa) and 0.100 M formic acid (HCOOH); (b) a solution that is 0.510 M pyridine (C5H5N) and 0.450 M in pyridinium chloride (C5H5NHCl).

Answer 1

Use the Henderson-Hasselbalch equation for both a and b:   pH = pKa + log [[salt]/ [acid] You need to look up the pKa of formic acid and pyridinium chloride. 

a)     pH = 3.75 + log 0.0.250 / 0.100

pH = 3.75 + 0.398 = 4.15

a)     pH= 5.25 + log [0.510] /[ 0.450]. In this case, the pyridinium chloride ionizes to the pyridinium ion that acts as the acid, and the pyridine acts as the “salt”. 

pH = 5.25 + 0.0544 = 5.30