Spreadsheet #18:  Titration of a Weak Acid with a Strong Base - Using the Ratio of  [A-] / [HA] to Determine Ka

[Back to Group 1 Assignments] [Back to Chem3020 home]

You will now fit the same titration data in a different manner to graphically evaluate pKa for acetic acid.  Recall from the last exercise

Equation 1:     Ka = [H+]*[A-] / [HA].

By taking the negative log of both sides and rearranging, we obtain

Equation 6:     pH  = pKa  +   log ( [A-] / [HA] ).

From Equation 2,  [A-]/[HA] = (Vb)/(Veq-Vb), we can obtain a linear relationship:

Equation 7:     pH  = pKa  +   log ( Vb / (Veq-Vb)).

Linear Eqn:    y  =  b +  m * x

Therefore, by plotting    log ( Vb / (Veq-Vb))   as a function of  pH, a slope of 1 is obtained, and an the absolute value of both intercepts is equal to pKa!

Knowing the value of Veq from Spreadsheets #16  and #17, construct a plot of Equation 7 and fit the data with linear regression.  Report the values of both intercepts, the slope, and the values of  both pKa and Ka for acetic acid.  (Click here to download data: 18pHrati.)

Spreadsheet # 18
Titration Curves for Weak Acid - Strong Base
Plot of pH vs. log (A-/HA) to Determine pKa
Assume Volume of NaOH at equavalence, Veq, is 10.00 mL:
.	Veq =	10.00	mL	.	.
#	Vb	pH	LOG(Vb/(Veq-Vb))	Linear Regression for Points 20-50
1	0.004	2.890	.	Slope:	.
2	0.048	2.964	.	Y-intercept:	.
3	0.094	3.038	.	R^2:	.
4	0.142	3.112	.	X-intercept:	.
5	0.194	3.186	.	pKa =	.
6	0.251	3.260	.	Ka =	.
7	0.314	3.335	.
...	...	...	...
82	10.001	8.892	.
83	10.001	8.966	.
84	10.002	9.040	.
85	10.002	9.114	.

Excel Programming Tips:

Remember: the log of zero and’or the log of a negative number is undefined in the Excel world. So, avoid ERROR warnings accordingly.