Question

the Henderson Hasslebach equation describes the behavior of a weak acid at a given pH.   By knowing the pKa of the acid, it is possible to calculate (in a pure water/acid) system, how many molecules of the given acid group are actually in the ionized state.    The Henderson Hasslebach equation:  pH = pKa +log10(CA‐/CAH) where Ca‐ is the concentration of the ionized acid and CAH is the concentration of the protonated acid.     Citric acid has 3 separate ionizable carboxylic groups with pKas of 3.13. 4.76 and 6.40.    Using absolute referencing, create a table that describes the percent ionization for each of the three acid groups of citric acid groups in the pH range of 3.0 to 8, in ncrements of 0.2 pH.  Format the table such that the columns are the degree of ionization for each acid and the rows are the pHs. Note: % Ionized = Ca‐/(Ca‐ + CAH)

Answer 1

Citric acid pka1=3.13

Pka2=4.76

Pka3=6.40

We need to create table with pH values ranging from 3 to 8 in increments of 0.2.

% ionized=Ca-/Ca-+CaH

Calculations done for pH=3 to 3.6 rest can be done similarly.

pH=pKa+log CA-/CAH

or,log CA-/CAH=pH-Pka

or,CA-/CAH=10^(pH-pKa)

or,CA-/CAH +1=10^(pH-pKa)+1

or,CA-+CAH/CAH=10^(pH-pKa)+1

or,CAH/CA+CAH=1/(10^(pH-pKa)+1)

Or,% ionizated=1/(10^(pH-pKa)+1)……………………(A)

Equation A can be used to calculate % ionized

pH	Pka1(% ionized)	Pka2(% ionized=)	Pka3(% ionized=)
3	0.57	0.59	0.99
3.2	0.46	0.97	0.99
3.4	0.35	0.95	0.99
3.6	0.25	0.93	0.99
3.8	0.17
4.0
4.2
4.4
4.6