Is this the same α as “fraction dissociated” for pure weak acid HA in water?

Not unless the pH is tied to that solution’s equilibrium. Here pH is an independent control (bath), giving the Henderson composition ratio. In pure HA, pH is coupled to concentration and α follows from charge balance, not this fixed-pH formula alone.

Why use base-10 exponentials?

Convention in aqueous chemistry: pH and pKa are defined with log₁₀, so the ratio [HA]/[A⁻] becomes a power of ten.

Does activity coefficient γ matter?

At high ionic strength, activities deviate from concentrations and effective pKa shifts. This page uses the textbook ideal limit.

Acid Dissociation α(pH)

For a single weak-acid/conjugate-base pair HA ⇌ H⁺ + A⁻ at fixed pH (fast acid–base exchange, dilute ideal solution), the fraction of the analytical pool present as A⁻ is α = [A⁻]/([HA]+[A⁻]). From Ka = a_H+ a_A− / a_HA with activities approximated by concentrations and a fixed hydrogen activity set by the bath, rearrangement gives α = 1/(1 + 10^(pKa−pH)). When pH = pKa, α = ½ — the half-equivalence composition familiar from titration curves. The simulator plots α and 1−α versus pH for an adjustable pKa and marks the current (pH, α) point plus the half-point at (pKa, ½). It does not compute pH self-consistently in a beaker of pure weak acid (that requires charge balance and water autoprotolysis) and omits ionic-strength corrections and polyprotic systems.

Who it's for: General chemistry alongside Henderson–Hasselbalch and titration labs; pairs with the buffer-solution page.

Key terms

degree of dissociation
pKa
titration
half-equivalence
conjugate base
Sigmoid

Markers: current pH (white) and half-equivalence at pH = pK_a (red).

Live graphs

How it works

For a weak acid / conjugate base pair at fixed pH (fast exchange, ideal dilute limit), the fraction deprotonated obeys α = [A⁻]/([HA]+[A⁻]) = 1/(1 + 10^(pKa−pH)). At pH = pKa exactly half is A⁻ (α = ½) — the half-equivalence composition on a titration curve. This page plots α and 1−α vs pH for a chosen pKa; it is a thermodynamic ratio, not a kinetics of dissociation in pure acid without specifying pH.

Key equations

α = 1 / (1 + 10^(pK_a − pH))

pH = pK_a ⇒ α = ½ (half A⁻, half HA)

Frequently asked questions

Is this the same α as “fraction dissociated” for pure weak acid HA in water?: Not unless the pH is tied to that solution’s equilibrium. Here pH is an independent control (bath), giving the Henderson composition ratio. In pure HA, pH is coupled to concentration and α follows from charge balance, not this fixed-pH formula alone.
Why use base-10 exponentials?: Convention in aqueous chemistry: pH and pKa are defined with log₁₀, so the ratio [HA]/[A⁻] becomes a power of ten.
Does activity coefficient γ matter?: At high ionic strength, activities deviate from concentrations and effective pKa shifts. This page uses the textbook ideal limit.

Other simulators in this category — or see all 55.

View category →

NewSchool

Born–Haber Cycle (NaCl & CaO)

Step ΔH° table and enthalpy ladder; lattice energy U = Σ(gas-ion steps) − ΔH_f°.

Launch Simulator

NewSchool

Frost Circle & Aromaticity (4n+2)

Inscribed n-gon on Hückel π energies; π count vs 4k+2 / 4k; Aufbau filling.

Launch Simulator

NewSchool

Henry's Law (Gas Solubility)

p = k_H c with k_H(T) from ΔH; c vs T at fixed p (ideal-dilute sketch).

Launch Simulator

NewUniversity / research

Gray–Scott Patterns

Reaction–diffusion u,v; coral / mitosis / worms / spirals; D_u, D_v, Δt.

Launch Simulator

NewSchool

Gibbs Free Energy

ΔG = ΔH − TΔS; sign vs spontaneity at constant p,T (no Q or K).

Launch Simulator

NewSchool

Hückel π-MO (Butadiene & Benzene)

Secular matrix H = αI + βA; eigen-energies and LCAO maps on the π skeleton.

Launch Simulator

Acid Dissociation α(pH)

Live graphs

How it works

Key equations

Frequently asked questions

More from Chemistry

Born–Haber Cycle (NaCl & CaO)

Frost Circle & Aromaticity (4n+2)

Henry's Law (Gas Solubility)

Gray–Scott Patterns

Gibbs Free Energy

Hückel π-MO (Butadiene & Benzene)

Acid Dissociation α(pH)

Live graphs

How it works

Key equations

Frequently asked questions