pOH Calculator
The base rail of the ladder: hydroxide in, pOH out, and the pH that must sit on the other end of the see-saw.
pOH Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Strong-base arithmetic — weak bases need equilibrium
- Concentration stands in for activity; pKw pinned at 25 °C
In short: 0.001000 mol/L of NaOH, one hydroxide per molecule: [OH⁻] = 0.001000 and pOH = 3.000000. The ladder pins pH at 11.000000 — the two always sum to 14.000000 at 25 °C. The water balance prices the other rail: [H⁺] = Kw/[OH⁻] = 1.000e-11 mol/L, six rungs below neutral and nothing like zero. Barium hydroxide donates TWO hydroxides: 0.005000 mol/L of it reads pOH 2.000000.
Formula
pOH = −log10[OH⁻] · [OH⁻] = C·hydroxides · pOH + pH = 14.00 at 25 °C · [H⁺] = Kw/[OH⁻]
pOH is the pH ladder’s other rail, measured on hydroxide instead of hydronium — same log ruler, opposite end of the see-saw. Strong bases donate every hydroxide they are given credit for, so the concentration multiplies by the hydroxide count. The rails are pinned together by water’s ion product: 14.00 apart at 25 °C.
Worked Example
- Enter the base’s molar concentration.
- Enter how many hydroxides each molecule donates.
- Read pOH and the pH waiting on the other rail.
- Optionally type a pOH and read the concentration behind it.
Defaults: 0.001000 mol/L NaOH → pOH 3.000000, pH 11.000000. Ba(OH)₂ at 0.005000 mol/L: pOH 2.000000. Typed pOH 2.3 → [OH⁻] = 5.012e-3 mol/L.
Strengths & Limits Of This Model
Where this engine is strong
- Hydroxide counted per molecule, explicitly
- pH partner and water balance computed, not assumed
Where it stops
- No ionic-strength correction
- No weak-base equilibrium
Practical Use Cases
Strong-base prep
lye and limewater, priced
Cleaning validation
caustic strengths on the rail
Reading the other rail
pH from hydroxide directly
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
pOH Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Why a separate page for pOH at all?
Because bases are dosed and measured in hydroxide, not in the hydronium they suppress. A caustic washer’s label speaks OH⁻; converting through pH first is a detour with a hand-waving step. This page owns the rail directly: hydroxide in, pOH out, and the ladder partner computed, not assumed.
How is this different from the pH page?
The question, not the arithmetic. The pH page starts from acid and counts protons; this page starts from base and counts hydroxides. The two meet on the same ladder — 14.00 apart at 25 °C — but each computes from its own side, which is how lab work actually flows.
Why does barium hydroxide count twice?
Because each Ba(OH)₂ carries two hydroxides and both land in the water: 0.005000 mol/L of it is 0.010000 mol/L of OH⁻, and pOH reads the hydroxide. The multiplier is the molecule’s own anatomy — calcium hydroxide plays the same game, just less soluble.
What does pOH 7 mean?
Neutral water — hydroxide alone at 1.0e-7 mol/L with hydronium equal to it. The middle rung is the same fact from both rails. Below 7 the solution walks on the base side of the see-saw; above 7 (yes, ABOVE for pOH) it is acidic — the rails run opposite, which is the classic exam trap.
Can pOH be negative?
Yes, for concentrated strong bases: 10 mol/L NaOH has pOH −1. The log ruler does not stop at its printed rungs, and neither does the page. The 0-to-14 picture is the aqueous room-temperature habit; the arithmetic keeps going, honestly.
How does the inverse door work here?
Type a pOH and the page converts back: 10 to the minus-pOH. A typed 2.3 corresponds to 5.012e-3 mol/L of hydroxide — a caustic rinse’s strength from its rail reading. The pH partner and the water balance come along for the ride.
Why is the hydronium figure so small on strong bases?
Because the water balance divides Kw by the hydroxide: in 0.001000 mol/L NaOH, hydronium is 1.000e-11 mol/L. Small is not zero — the ions never both vanish — and the balance is why strong base and strong acid cannot coexist at strength in the same beaker.
Where do weak bases fit?
Ammonia and friends donate hydroxide GRUDGINGLY — the molecule pulls a proton from water only partly, so the hydroxide count is an equilibrium outcome, not the formula’s arithmetic. This page prices STRONG bases; weak-base chemistry needs its equilibrium constant, which is a later chapter of this category.