Question

You wish to prepare an HC2H3O2 buffer with a pH of 4.24. If the pKa of is 4.74, what ratio of C2H3O2 /HC2H3O2 must you use?

Answer 1

To solve this problem, we can use the Henderson-Hasselbalch Equation which relates the pH to the measure of acidity pKa. The equation is given as:

pH = pKa + log ([base]/[acid])                ---> 1

Where,

[base] = concentration of C2H3O2 in molarity or moles

[acid] = concentration of  HC2H3O2 in molarity or moles

 

For the sake of easy calculation, let us assume that:

[base] = 1

[acid] = x

Therefore using equation 1,
4.24 = 4.74 + log (1 / x) 

log (1 / x) = - 0.5

1 / x = 0.6065 

x = 1.65

The required ratio of C2H3O2 /HC2H3O2 is 1:1.65 or 3:5.