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Calcolatore pH e pOH

Calcola il pH e il pOH delle tue soluzioni. Strumento di chimica essenziale per analisi di acidi e basi. Ottieni risultati precisi gratis online.

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Which value do you already know?

Enter one known acid-base value and the calculator returns pH, pOH, [H+], [OH-] and the classification, with the formula and the working. The arithmetic runs in your browser — no AI model decides the number.

Known value

Starting from with

Decimals, scientific notation and the textbook form all work: 0.001, 1e-3 or 10^-3.

Results update as you type — the button is only there if you prefer to press one.
Try an example

Assumes 25 °C, where pH + pOH = 14.

Step-by-step working

Written out so you can hand in the working, not just the number. Check each line against your own arithmetic before you copy it.

Find pH, pOH, [H+] and [OH-] for chemistry homework and lab problems. Pick the value you already know, type it in — 1e-3 and 10^-3 both work — and press Calculate pH. You get all four values, an acidic / neutral / basic classification, and the formula with each step of the working. Everything runs in your browser at 25 °C, where pH + pOH = 14. No account, no upload, nothing saved.

How to use the pH calculator

Pick the value you know

Choose pH, pOH, [H+] or [OH-] — whichever one the question gives you. The formula for that route is shown next to the selector so you can see which conversion is about to run.

Type the value and press Calculate pH

Decimals, scientific notation such as 1e-3, and the textbook form 10^-3 are all accepted. Concentrations take M or mol/L, which are the same unit. Nothing is sent anywhere — the logarithms run in your browser.

Read all four values and the steps

You get pH, pOH, [H+], [OH-] and the acidic, neutral or basic classification, plus the formula, the substitution and the answer written out line by line so you can hand in the working.

The four values, and why one of them is enough

pH, pOH, [H+] and [OH-] are four ways of describing the same solution. Fix one and the other three follow, which is what makes this a conversion problem rather than a measurement.

pH and pOH

Both are negative base-10 logarithms, so they compress a range spanning fourteen powers of ten into readable single-digit numbers. At 25 °C they always add to 14, which is why subtraction converts between them.

[H+] and [OH-]

The actual concentrations, in moles per litre. They are recovered from a p-value by raising ten to its negative power, and their product is fixed at 1.0 × 10^-14 at 25 °C.

The classification

Acidic below pH 7, basic above it, neutral when [H+] equals [OH-]. This is a consequence of the numbers rather than a separate measurement, so the calculator derives it instead of asking for it.

Where pH questions usually go wrong

The formulas are short. Almost every lost mark in this topic comes from notation or from the direction of the conversion, not from the chemistry.

Notation and sign errors

The minus sign in pH = -log10([H+]) is doing real work: without it every acidic solution reports a negative pH. The other classic is entering 10^-3 as -3, or dropping the exponent entirely and typing 1.

  • -log10(1e-3) = 3, not -3
  • 10^-3 = 0.001, not -3
  • log10, not ln
  • [H+] is always positive

Going the wrong way

Subtracting from 14 converts pH to pOH and back, but it does not convert a p-value into a concentration — that step needs a power of ten. Mixing the two produces answers that look plausible and are off by orders of magnitude.

  • pOH = 14 - pH
  • [H+] = 10^-pH
  • pOH 3 means pH 11, not [OH-] 11
  • Check: does acidic give pH under 7?

Come funziona

  1. Inserisci la concentrazione degli ioni idrogeno $[H^+]$.
  2. Seleziona l'unità di misura della concentrazione.
  3. Premi "Calcola pH".
  4. Determina se la soluzione è acida, neutra o basica dal risultato.

pH calculator examples

Every example below is computed by the same code the calculator runs, so the numbers and the working match exactly what you get when you type your own value in.

Example 1: Find pH from hydrogen ion concentration

If [H+] = 1 × 10^-3 M, then pH = 3. The concentration goes straight into the logarithm, and pOH follows by subtraction.

Formula
pH = -log10([H+])
Given
[H+] = 1.0 × 10^-3 M
Calculation
pH = -log10(1.0 × 10^-3) = 3.00
Then — pOH
pOH = 14 - 3.00 = 11.00
Then — [OH-]
[OH-] = 10^-11.00 = 1.0 × 10^-11 M
Answer
pH = 3.00, acidic

Full readout: pH 3.00, pOH 11.00, [H+] = 1.0 × 10^-3 M, [OH-] = 1.0 × 10^-11 M — acidic.

Example 2: Find pOH from pH

If pH = 4.5 at 25 °C, then pOH = 14 - 4.5 = 9.5. No logarithm is needed for the conversion itself, only for the concentrations that follow.

Formula
pOH = 14 - pH, [H+] = 10^-pH, [OH-] = 10^-pOH
Given
pH = 4.50
Step 1 — pOH
pOH = 14 - 4.50 = 9.50
Step 2 — [H+]
[H+] = 10^-4.50 = 3.2 × 10^-5 M
Step 3 — [OH-]
[OH-] = 10^-9.50 = 3.2 × 10^-10 M
Answer
pH = 4.50, acidic

Full readout: pH 4.50, pOH 9.50, [H+] = 3.2 × 10^-5 M, [OH-] = 3.2 × 10^-10 M — acidic.

Example 3: Find [H+] from pH

If pH = 2, then [H+] = 10^-2 = 0.01 M. Raising ten to the negative pH reverses the logarithm that produced it.

Formula
pOH = 14 - pH, [H+] = 10^-pH, [OH-] = 10^-pOH
Given
pH = 2.00
Step 1 — pOH
pOH = 14 - 2.00 = 12.00
Step 2 — [H+]
[H+] = 10^-2.00 = 1.0 × 10^-2 M
Step 3 — [OH-]
[OH-] = 10^-12.00 = 1.0 × 10^-12 M
Answer
pH = 2.00, acidic

Full readout: pH 2.00, pOH 12.00, [H+] = 1.0 × 10^-2 M, [OH-] = 1.0 × 10^-12 M — acidic.

Example 4: Find pH from hydroxide ion concentration

If [OH-] = 1 × 10^-4 M, the logarithm gives pOH = 4 first, and pH = 14 - 4 = 10 makes the solution basic.

Formula
pOH = -log10([OH-])
Given
[OH-] = 1.0 × 10^-4 M
Calculation
pOH = -log10(1.0 × 10^-4) = 4.00
Then — pH
pH = 14 - 4.00 = 10.00
Then — [H+]
[H+] = 10^-10.00 = 1.0 × 10^-10 M
Answer
pH = 10.00, basic

Full readout: pH 10.00, pOH 4.00, [H+] = 1.0 × 10^-10 M, [OH-] = 1.0 × 10^-4 M — basic.

Example 5: Find pH from pOH

If pOH = 3, then pH = 14 - 3 = 11. A low pOH means a high pH, which catches students out more often than any other step here.

Formula
pH = 14 - pOH, [OH-] = 10^-pOH, [H+] = 10^-pH
Given
pOH = 3.00
Step 1 — pH
pH = 14 - 3.00 = 11.00
Step 2 — [OH-]
[OH-] = 10^-3.00 = 1.0 × 10^-3 M
Step 3 — [H+]
[H+] = 10^-11.00 = 1.0 × 10^-11 M
Answer
pH = 11.00, basic

Full readout: pH 11.00, pOH 3.00, [H+] = 1.0 × 10^-11 M, [OH-] = 1.0 × 10^-3 M — basic.

Example 6: Check a neutral solution

Pure water at 25 °C has [H+] = 1 × 10^-7 M, which gives pH 7.00 and pOH 7.00 with [H+] equal to [OH-] — the definition of neutral.

Formula
pH = -log10([H+])
Given
[H+] = 1.0 × 10^-7 M
Calculation
pH = -log10(1.0 × 10^-7) = 7.00
Then — pOH
pOH = 14 - 7.00 = 7.00
Then — [OH-]
[OH-] = 10^-7.00 = 1.0 × 10^-7 M
Answer
pH = 7.00, neutral

Full readout: pH 7.00, pOH 7.00, [H+] = 1.0 × 10^-7 M, [OH-] = 1.0 × 10^-7 M — neutral.

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Frequently Asked Questions

What does a pH calculator do?
A pH calculator helps convert between pH, pOH, hydrogen ion concentration, and hydroxide ion concentration for acid-base chemistry problems. You enter whichever of the four values the question gives you, and it returns the other three along with an acidic, neutral or basic classification. Because the four quantities are linked by fixed formulas at 25 °C, one known value is always enough to determine the rest.
What is the formula for pH?
The formula is pH = -log10([H+]), where [H+] is the hydrogen ion concentration in moles per litre. Reversing it gives [H+] = 10^-pH, so a pH of 3 corresponds to a hydrogen ion concentration of 1.0 × 10^-3 M. The negative sign is what keeps everyday acidic solutions on a positive, readable scale instead of reporting them as negative numbers.
How are pH and pOH related?
At 25 °C, pH + pOH = 14. If you know one value, you can subtract it from 14 to find the other. That relationship comes from the ion product of water, Kw = [H+] × [OH-] = 1.0 × 10^-14, so it holds for any aqueous solution at that temperature. Kw shifts with temperature, which is why this calculator states 25 °C rather than treating 14 as universal.
What pH is neutral?
At 25 °C, a neutral aqueous solution has a pH close to 7. Values below 7 are acidic and values above 7 are basic. Strictly, neutral means the hydrogen and hydroxide concentrations are equal — both 1.0 × 10^-7 M at 25 °C — and pH 7 is the consequence rather than the definition. Decimal arithmetic sometimes produces 6.999999 for an exactly neutral solution, so anything that rounds to 7.00 is reported as neutral here.
Can this calculator help with chemistry homework?
Yes. EduSolver shows the formula and steps so students can understand acid-base calculations instead of only seeing the final answer. Each result lists the formula used, the values substituted into it, and every intermediate conversion, so the working can be checked line by line against your own. Weak acids and bases, buffers and titration curves are outside this tool — the EduSolver AI chemistry tutor covers those.

Stuck on the chemistry around the pH value?

EduSolver's AI tutor reads the whole problem, handles weak acids, buffers and titrations, and explains each step — then turns it into flashcards for the test.