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pH Calculator

The pH of a solution equals the negative base-10 logarithm of its hydrogen ion concentration: pH = -log10[H+]. A pH below 7 indicates an acidic solution, a pH above 7 indicates a basic (alkaline) solution, and a pH of exactly 7 is neutral. This calculator handles two common conversions: enter a hydrogen ion concentration to find the pH, or enter a pH value to find the corresponding ion concentrations.

Quick Answer

A hydrogen ion concentration of 1.0 x 10^-4 mol/L gives a pH of 4.00 and a pOH of 10.00, making the solution acidic. A pH of 9.0 corresponds to [H+] = 1.0 x 10^-9 mol/L and [OH-] = 1.0 x 10^-5 mol/L.

[H+] Concentration to pH

Enter the hydrogen ion concentration in mol/L (e.g., 0.001 for 10^-3).

pH to Ion Concentrations

Enter a pH value between 0 and 14.

Common Examples

Input Result
[H+] = 0.001 mol/L pH = 3.00, pOH = 11.00 (acidic)
[H+] = 0.0000001 mol/L pH = 7.00, pOH = 7.00 (neutral)
pH = 2.0 [H+] = 1.00 x 10^-2, [OH-] = 1.00 x 10^-12
pH = 11.5 [H+] = 3.16 x 10^-12, [OH-] = 3.16 x 10^-3
[H+] = 0.05 mol/L pH = 1.30, pOH = 12.70 (acidic)

How It Works

The Formula

The pH scale measures how acidic or basic a solution is, based on the concentration of hydrogen ions (H+) in moles per liter:

pH = -log10([H+])

Where [H+] is the hydrogen ion concentration in mol/L (molarity). The pH scale typically ranges from 0 to 14, though values outside this range are possible for very concentrated solutions.

Complementary Relationships

The pOH measures hydroxide ion concentration and is related to pH by:

pOH = -log10([OH-])

pH + pOH = 14 (at 25 degrees Celsius)

This relationship comes from the ion product constant of water: [H+] x [OH-] = 1.0 x 10^-14 at 25 degrees Celsius (Kw).

Reverse Calculations

To find ion concentrations from pH:

[H+] = 10^(-pH)

[OH-] = 10^(-pOH) = 10^(-(14 - pH))

Classification

  • pH < 7: acidic (higher H+ concentration)
  • pH = 7: neutral (pure water at 25 degrees Celsius)
  • pH > 7: basic/alkaline (higher OH- concentration)

Each whole number change in pH represents a tenfold change in hydrogen ion concentration. A solution with pH 3 has 10 times more H+ ions than a solution with pH 4, and 100 times more than pH 5.

Common pH Values

  • Battery acid: ~1.0
  • Lemon juice: ~2.0
  • Vinegar: ~2.9
  • Coffee: ~5.0
  • Pure water: 7.0
  • Baking soda solution: ~8.3
  • Ammonia: ~11.0
  • Bleach: ~12.5

Worked Example

For a solution with [H+] = 0.001 mol/L (which is 1.0 x 10^-3): pH = -log10(0.001) = -log10(10^-3) = 3.00. pOH = 14 - 3.00 = 11.00. [OH-] = 10^-11 = 1.0 x 10^-11 mol/L. Since pH < 7, the solution is acidic. For the reverse, starting with pH = 9.0: [H+] = 10^-9.0 = 1.0 x 10^-9 mol/L. pOH = 14 - 9.0 = 5.0. [OH-] = 10^-5.0 = 1.0 x 10^-5 mol/L. Since pH > 7, the solution is basic.

Related Calculators

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Calculate the molar mass of any chemical compound by entering its formula. Shows element breakdown with atomic masses.

Moles Calculator

Convert between mass, moles, and molar mass using the mole formula n = mass / molar mass.

Frequently Asked Questions

What does pH stand for?
The abbreviation pH stands for 'potential of hydrogen' (or 'power of hydrogen'). It was introduced by Danish chemist S.P.L. Sorensen in 1909 as a convenient way to express hydrogen ion concentration on a logarithmic scale, making it easier to work with the very small numbers involved in aqueous chemistry.
Why does the pH scale go from 0 to 14?
The range 0 to 14 comes from the ion product constant of water (Kw = 1.0 x 10^-14 at 25 degrees Celsius). Since pH + pOH = 14, a pH of 0 means [H+] = 1 mol/L, and a pH of 14 means [OH-] = 1 mol/L. Values outside this range are possible with very concentrated acids or bases, but 0 to 14 covers the vast majority of practical solutions.
Can pH be negative or greater than 14?
Yes, though it is uncommon in typical chemistry. A very concentrated strong acid can have a pH below 0 (for example, concentrated hydrochloric acid at 10 mol/L has a theoretical pH of -1). Similarly, very concentrated strong bases can exceed pH 14. These extreme values are mostly relevant in industrial chemistry.
What is the difference between pH and pOH?
pH measures the hydrogen ion [H+] concentration, while pOH measures the hydroxide ion [OH-] concentration. They are complementary: pH + pOH = 14 at 25 degrees Celsius. An acidic solution has a low pH and high pOH, while a basic solution has a high pH and low pOH.
Does temperature affect pH?
Yes. The ion product constant Kw changes with temperature, which shifts the neutral point. At 25 degrees Celsius, neutral water has pH 7.00. At higher temperatures, Kw increases, and the neutral pH drops slightly below 7. This calculator assumes standard conditions (25 degrees Celsius), which is the convention for most chemistry coursework and laboratory work.