pH Calculator
pH from hydrogen ion concentration and back, with acid or base verdict.
What it does
pH compresses huge concentration spans onto a small scale. Strong stomach acid near pH 2 holds ten million times the hydrogen ions of blood near pH 7.4, and the logarithm is what fits both on one ruler.
The tool above converts both directions at 25 C. Enter molarity for the pH, or enter pH for the molarity, with an acidic, neutral, or basic verdict beside every answer.
The default of 0.01 M converts to pH 2 (acidic), the classic strong acid bench point. Flipping to pH 7 returns 1e-7 M neutral, the reference point of pure water at room temperature.
The scale follows IUPAC Gold Book definitions with NIST standard buffer solutions behind commercial calibration. Both treat pH 7 as neutral only at 25 C; hot water neutrality sits lower.
How to use
- Pick the direction first.
- For concentration, enter molarity like 0.01.
- For pH, enter 0 to 14 like 7.
- Read pH and concentration together with the verdict.
- Recalibrate real meters with buffers before trusting decimals.
Formula
- [H+]
- Hydrogen ion molarity in mol/L.
- log10
- Base ten log compressing the scale.
- 25 C
- Reference temperature for neutral 7.
Strong monoprotic acids assumed, where each molecule frees one ion. Weak acids split partly and need their equilibrium constant instead.
Worked example
Example: 0.01 M to pH
Concentration 0.01 M, direction to pH.
- log10(0.01) = -2.
- Negate: -(-2) = 2.
- pH reads 2, below 7, so acidic.
- Reverse check: 10^-2 = 0.01 M back.
- Tool shows 2, 0.01 M, Acidic.
Reference table
| Sample | pH | Ion conc. |
|---|---|---|
| Stomach acid | about 2 | 0.01 M |
| Black coffee | about 5 | 1e-5 M |
| Pure water | 7 | 1e-7 M |
| Soap | about 10 | 1e-10 M |
| Bleach | about 13 | 1e-13 M |
Limits
- Strong acid model only; weak acids need Ka math.
- Neutral 7 holds at 25 C and drifts with temperature.
- Activity corrections matter in salty solutions.
- Meters need buffer calibration; strips read coarsely.
Common mistakes
- Reading pH 6 as twice pH 3 instead of a thousand times weaker.
- Entering percent instead of molarity.
- Assuming neutrality is always exactly 7 at any temperature.
- Trusting uncalibrated meter decimals.
- Mixing pH with pOH without converting.
Try it yourself
Quick checks. Answers sit tucked under each question.
pH of 0.001 M?
log10 is -3, so pH 3, acidic.
Conc at pH 7?
10^-7 = 1e-7 M, neutral.
Conc at pH 4?
10^-4 = 0.0001 M.
Questions
How do you calculate pH?
Minus log10 of hydrogen molarity. 0.01 M gives pH 2.
What does each pH unit mean?
Ten times the ions. pH 3 is ten times pH 4 and a hundred times pH 5.
Is 7 always neutral?
At 25 C, yes. Hotter water shifts neutral below 7.
Acid or base at pH 9?
Basic. Above 7 is basic, below is acidic.
Why assume strong acids?
They free essentially every ion, so concentration equals ion count. Weak acids need equilibrium math.
pH paper vs meter?
Paper reads whole units fast; calibrated meters read hundredths for lab work.