pH Calculator
pH compresses an enormous range of hydrogen ion concentrations into a small, manageable number scale using a logarithm — which is exactly why each single pH unit represents a tenfold change in actual acidity, not a small linear step.
Inputs
Result
4
pOH 10 · Acidic
How it works
pH = −log₁₀[H⁺]
How the ph calculator works
pH = −log₁₀[H⁺], where [H⁺] is hydrogen ion concentration in moles per litre.
pOH = 14 − pH (at standard conditions), and the solution is classified as acidic (pH < 7), basic (pH > 7), or neutral (pH = 7).
Worked example: [H⁺] = 0.0001 mol/L
- pH = −log₁₀(0.0001) = −(−4) = 4.
- pOH = 14 − 4 = 10.
- pH 4 is well below 7, so this solution is classified as acidic.
Common mistakes to avoid
Treating pH differences as linear rather than logarithmic
A solution with pH 3 isn't twice as acidic as one with pH 6 — it's a thousand times more acidic (10³, since each whole pH unit represents a tenfold change in hydrogen ion concentration), which is easy to underestimate without remembering the logarithmic scale.
Forgetting the negative sign in the pH formula
Since [H⁺] concentrations are typically small decimal numbers, their log₁₀ is negative — the formula's leading minus sign converts that negative log into the conventional positive pH value most people are familiar with.
Frequently asked questions
Why does a pH of 7 count as 'neutral'?
At pH 7, hydrogen ion concentration equals hydroxide ion concentration exactly, which is the defining condition for pure water at standard temperature — this balance point was adopted as the reference for 'neutral' on the scale.
Why is each pH unit a tenfold change rather than a simple linear step?
Because pH is defined using a base-10 logarithm — each decrease of 1 in pH corresponds to a tenfold increase in actual hydrogen ion concentration, which is what makes the scale able to compactly represent an enormous range of real acidity levels.
How is pOH related to pH?
At standard conditions (25°C), pH + pOH always equals 14 — knowing one directly gives the other through simple subtraction, since they represent complementary aspects of the same underlying water equilibrium.
Can pH go below 0 or above 14?
Yes, for sufficiently concentrated acids or bases, pH can technically extend beyond the commonly cited 0-14 range — that range is a practical, commonly encountered window rather than a strict mathematical boundary.
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