Write the Ka expression for acetic acid and formic acid used in Part B. In addit

Question

Write the Ka expression for acetic acid and formic acid used in Part B. In addition find the Ka values for acetic acid and formic acid and calculate their pKa values. Part B is noted as follows:

Prepare 0.100 L of each buffer in a 100 mL volumetric flask. Using a graduated cylinder measure the necessary volumes of acid, base, and water which you have calculated: Chemicals used: 0.50 M HC2H3O2, 0.50 M NaC2H3O2, 0.50 M HCOOH, 0.50 M NaCOOH

Buffer 1: Prepare 0.100 L of a pH = 3.74 buffer with a total buffer strenth of 0.100M

Buffer 2: Prepare 0.100 L of a pH = 4.04 buffer with a total buffer strenth of 0.100M

Buffer 3: Prepare 0.100 L of a pH = 4.44 buffer with a total buffer strenth of 0.100M

Buffer 4: Prepare 0.100 L of a pH = 5.04 buffer with a total buffer strenth of 0.100M

I know the pKa values are 3.75 and 4.75, but I have NO idea how to get to that point from this information. Help please!

Explanation / Answer

Ch3COOH <==> CH3COO-   +   H+

Ka = [CH3COO-][H+]/[CH3COOH]

HCOOH = H+ + HCOO-

Ka = [H+][HCOO-]/[HCOOH]

pKa = -log Ka

or, Ka = 10^-pKa = 10^-3.75 = 1.78*10^-4 (for acetic acid)

     Ks = 10^-pKa = 10^-4.75 = 1.78 *10^-5 (for formic acid)

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