Phil asked in Science & MathematicsChemistry · 9 years ago

Calculate the pH of the following solutions?

I need a bit of additional help in solving these two problems. I can't seem to find the right answer. Help!

(a) 0.162 M in sodium formate (NaCHO2) and 0.325 M in formic acid (HCHO2)

(b) 0.205 M in pyridine (C5H5N) and 0.415 M in pyridinium chloride (C5H5NHCl)

Relevance
• A.S.
Lv 7
9 years ago

a)its an acidic buffer ...as it contains weak acid (formic acid) and salt of weak acid with strong base

pH = pKa + log [salt]/[acid]

Ka for formic acid = 1.8 X 10^-4

so pKa = -log Ka = - log 1.8X10^-4 = 3.745

[salt] = 0.162

[acid] = 0.325

pH = 3.745 + log 0.162/0.325 = 3.745 + log 0.498 = 3.745 - 0.303 = 3.442

b)its a basic buffer...as it has weak base (pyridine) and salt of this base with strong acid

pOH = pKb + log [salt] / [base]

Kb for pyridine = 1.5X10^-9

pKb = -log Kb = -log 1.5 X 10^-9 = 8.824

pOH = 8.824 + log 0.415/0.205 = 8.824 + log 2.024 = 8.824 + 0.306 = 9.13

pH = 14-pOH = 14-9.13 = 4.87