Question

The pK3 of formic acid is 3.75
A)what is the pH of a buffer in which formic acid and sodium formate have equimolar concentration?
B)what is the pH of a solution in which the sodium formate is 10M and the formic acid is 1M?

Answer 1

I don't believe formic acid has a pK3. You must mean pKa.
a. pH = pKa + log (base)/(acid)
Since base = acid then base/acid = 1; log 1 is zero and pH = pKa.

b. Use the Henderson-Hasselbalch equation above and substitute for base and acid then calculate pH.

Answer 2

Can you explain it better we JUST started this section and im having a hard time with knowing what to put where when it comes to the solutions. what is the acid and what is the base

Answer 3

a. pH = pKa + log(base)/(acid)
pH = 3.75 + log(x)/(x) (the problem just says base = acid so I've called that x.
pH = 3.75 + log(1)
pH = 3.75 + 0
pH = 3.75

b.
HCOOH = formic acid = acid = 1.0M
HCOONa = sodium formate = base = 10M
pH = 3.75 + log (10)/(1)
Solve for pH You should get 4.75.
Acids/bases are described with the Bronsted-Lowry theory as
HA = acid
HA + H2O ==> H3O^+ + A^-
HA is the acid and A^- is its conjugate base.
H2O is the base and H3O^+ is its conjugate acid.
How do you know which is which? The acid is the one with more atoms; the base is the one with fewer H atoms. Look at HA. HA and A. HA is the acid because it has more H atoms than the A. A is the base becasuse it has fewer H atoms than HA.
H2O/H3O^+. H3O^+ has more H atoms; it's the acid. H2O has fewer H atoms; it's the base.

Answer 4

Thank you :) this helped a lot!