Question

You are interested in using formc acid ( Pka = 3.75) as a buffer for your biochemical experiment

b) you decide to use formic acid acid to make 1.5L od 70mM buffer at the pH you selected in part a. Determine the concentrations of formic acid and its conjugate base that are required?

Answer 1

Use Henderson-Hasselbach equation , to determine the ratio of concentration of conjugate acid and base in the buffer

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

acid=formic acid( HCOOH)

conjugate base=formate ion(HCOO-)

As pH in part a not mentioned, let me calculate with an arbitrary pH value say pH=2.0

2.0=3.75+log [HCOO-]/[HCOOH]

2.0-3.75= log [HCOO-]/[HCOOH]

-1.75= log [HCOO-]/[HCOOH]

10^(-1.75)= [HCOO-]/[HCOOH]

0.018= [HCOO-]/[HCOOH]

Also concentration of buffer can be calculated by the formula, concentration==molarity*volume=70mM*1.5L=70*10^-3 mol/L*1.5L=105*10^-3 moles=0.105 moles

Total moles in buffer=0.105 moles

We have, 0.018= [HCOO-]/[HCOOH]

So , [HCOO-]+[HCOOH]=0.105 moles

[HCOO-]=0.105-[HCOOH]

Or, 0.018=0.105-[HCOOH]/[HCOOH]

OR, 0.018[HCOOH]= 0.105-[HCOOH]

OR, 0.018[HCOOH]+[HCOOH]= 0.105

OR,1.018[HCOOH]=0.105

OR,[HCOOH]=0.105/1.018=0.103 MOLES

[HCOO-]=0.105-0.103=0.002 MOLES

[HCOOH]=0.103MOLES/TOTAL VOLUME=0.103MOLES/1.5L=0.069 M

[HCOO-]=0.002MOLES/1.5L=0.0013 M

CONCENTRATION OF FORMIC ACID=0.069M

CONC OF CONJ BASE=0.0013 M