- 1). Find the pKa of the acid for which you wish to calculate the Ka. Numerous pKa data tables are available online. As an illustrative example, boric acid, B(OH)3, exhibits a pKa of 9.23.
- 2). Change the sign of the pKa value from positive to negative.
- 3). Calculate Ka by taking the antilog of the negative pKa value. On most calculators, the antilog button is represented by 10^x, or 10 raised to the power of x, where x represents the number you have entered into the calculator. Continuing the example from step 1, 10^(-9.23) = 0.000 000 000 923, or, in scientific notation, 9.23 x 10^(-10).
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