4.757
0.0000000006
9.243
4.757
0.0000000006
9.243
The pKa Calculator converts the acid dissociation constant (Ka) to pKa and also computes the corresponding Kb and pKb for the conjugate base. The pKa is a fundamental parameter in acid-base chemistry that indicates the strength of an acid: the lower the pKa, the stronger the acid.
Enter the Ka value to instantly obtain the pKa, along with the conjugate base dissociation constant (Kb) and its pKb. This tool is used in chemistry, biochemistry, and pharmacology for understanding acid strength, predicting ionization states, and selecting appropriate buffers.
The pKa is defined as the negative base-10 logarithm of the acid dissociation constant:
pKa = -log10(Ka)
The conjugate base dissociation constant (Kb) is related to Ka through the water autoionization constant:
Ka x Kb = Kw = 1.0 x 10⁻¹⁴
Therefore: Kb = Kw / Ka and pKb = 14 - pKa
A small Ka (large pKa) indicates a weak acid that does not dissociate easily. A large Ka (small or negative pKa) indicates a strong acid that dissociates readily.
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Acetic acid has a pKa of approximately 4.76, making it a weak acid. Its conjugate base (acetate) has pKb = 9.24.
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Carbonic acid (first dissociation) has pKa approximately 6.37. The bicarbonate/carbonic acid system is the primary blood buffer.
The pKa indicates the strength of an acid. A lower pKa means a stronger acid (more readily donates protons), while a higher pKa means a weaker acid. Strong acids like HCl have negative pKa values, while weak acids like acetic acid have pKa values between 2 and 12. The pKa also equals the pH at which half of the acid molecules are dissociated.
For a conjugate acid-base pair in water at 25 degrees Celsius, pKa + pKb = 14 (which equals pKw). This means a strong acid (low pKa) has a weak conjugate base (high pKb), and vice versa. This relationship is fundamental to understanding how acid-base pairs behave in solution.
In pharmacology, the pKa of a drug determines what fraction is ionized versus un-ionized at a given physiological pH. Un-ionized molecules cross cell membranes more easily, so the pKa determines drug absorption, distribution, and excretion. For example, aspirin (pKa 3.5) is mostly un-ionized in the acidic stomach, facilitating absorption there.
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