Function protonsDonatedPerMolecule

Mean number of protons an acid has donated at a given pH.

For an acid HₙA with stepwise dissociation constants K₁…Kₙ, the relative abundance of the species that has lost j protons is

Rⱼ = (Π_{k≤j} K_k) / [H⁺]ʲ      →      log₁₀ Rⱼ = j·pH − Σ_{k≤j} pKa_k
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and the mean protons donated is the abundance-weighted mean of j:

f(pH) = Σ j·Rⱼ / Σ Rⱼ
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This is Henderson–Hasselbalch generalised to a polyprotic acid. It gives the number of equivalents one mole of the acid delivers at this pH, which is what the charge balance needs. We work in the exponent domain and subtract the max so that strong acids (pKa ≪ 0) and very high pH can't overflow.

Sanity checks, all covered by tests:

• lactic (pKa 3.86) at pH 5.40 → 0.9722
• phosphoric (2.148 / 7.198 / 12.32) at pH 5.40 → 1.0151
• citric (3.128 / 4.761 / 6.396) at pH 5.40 → 1.9015
• hydrochloric (strong) → 1.0000 at every brewing pH