pKa Calculator

pKa is the negative logarithm of the acid dissociation constant (Ka). It's crucial in chemistry because it indicates acid strength on a manageable scale. Lower pKa values (0-2) indicate strong acids, while higher values (4-14) indicate weak acids.

Lower pKa = stronger acid. For example, HCl (pKa ≈ -7) is much stronger than acetic acid (pKa = 4.76). This inverse relationship exists because pKa = -log(Ka), and stronger acids have larger Ka values.

Ka is the actual acid dissociation constant (often very small numbers like 1.8 × 10⁻⁵), while pKa is the negative logarithm of Ka (manageable numbers like 4.76). They contain the same information in different forms.

HCl: -7, H₂SO₄: -3, Acetic acid: 4.76, Carbonic acid: 6.35, Ammonia (NH₄⁺): 9.25, Water (H₃O⁺): 15.7. Strong acids have negative pKa values, weak acids have positive pKa values between 2-7.

The Henderson-Hasselbalch equation uses pKa to calculate buffer pH: pH = pKa + log([A⁻]/[HA]). Buffers work best when pH ≈ pKa (within ±1 pH unit), making pKa essential for buffer design.

Yes, very strong acids have negative pKa values. For example, HCl has pKa ≈ -7, and H₂SO₄ has pKa ≈ -3. Negative pKa indicates Ka > 1, meaning the acid dissociates almost completely.

For conjugate acid-base pairs, pKa + pKb = 14 at 25°C. This means if you know the pKa of an acid, you can calculate the pKb of its conjugate base, and vice versa.