To turn pH into hydrogen ion concentration, raise 10 to the power of minus the pH: [H+] = 10^-pH mol/L. A solution at pH 3.5 has [H+] = 10^-3.5 = 3.16 × 10^-4 mol/L. Each one-unit drop in pH means ten times more hydrogen ions, so pH 2.5 holds 3.16 × 10^-3 mol/L.
Students working acid-base problems, lab technicians checking a buffer, and pool or aquarium keepers reading a meter all meet the same step: a pH reading has to become a concentration before it can be used in an equilibrium expression or a dilution. Type a pH and the page returns [H+] in scientific notation, the same value as a plain decimal when it is large enough to read that way, the pOH, the hydroxide concentration and whether the solution is acidic, neutral or basic. The working is printed underneath so the exponent arithmetic can be copied into a lab report. One limit: pOH and [OH-] use pKw = 14, which is exact only at 25 °C; at 37 °C pKw is closer to 13.6, so the hydroxide figures shift while [H+] does not.
Type the measured or given pH, including decimals such as 4.75. Values from -2 to 16 are accepted.
The headline shows the hydrogen ion concentration in mol/L in scientific notation, with a decimal form for values of 0.0001 M and above.
The pOH and hydroxide concentration follow from pH + pOH = 14 at 25 °C.
The lines under the result show each power-of-ten step with your numbers filled in.
Result Hydrogen ion concentration [H+]: 1.00 × 10^-7 mol/L
[H+] as a decimal (mol/L): 1.00 × 10^-7 mol/L
pOH: 7
Hydroxide concentration [OH-]: 1.00 × 10^-7 mol/L
Solution is: Neutral
At 25 °C neutral water holds 1.0 × 10^-7 mol of hydrogen ions per litre, the same as hydroxide.
Result Hydrogen ion concentration [H+]: 3.16 × 10^-4 mol/L
[H+] as a decimal (mol/L): 0.000316
pOH: 10.5
Hydroxide concentration [OH-]: 3.16 × 10^-11 mol/L
Solution is: Acidic
10^-3.5 is 3.16 × 10^-4 mol/L, about 3,160 times the hydrogen ion level of neutral water.
Result Hydrogen ion concentration [H+]: 1.00 × 10^-12 mol/L
[H+] as a decimal (mol/L): 1.00 × 10^-12 mol/L
pOH: 2
Hydroxide concentration [OH-]: 1.00 × 10^-2 mol/L
Solution is: Basic
A 0.01 M sodium hydroxide solution has pOH 2, so [H+] falls to 1 × 10^-12 mol/L.
[H+] = 10^(-pH); pOH = 14 - pH; [OH-] = 10^(-pOH)
pH[H+]pOH[OH-]pH is defined as -log10[H+], so inverting it is an antilog: [H+] = 10^-pH. Water's ion product Kw = [H+][OH-] = 1.0 × 10^-14 at 25 °C, which gives pH + pOH = 14 and lets the hydroxide concentration be found from the same reading. Strictly, a pH meter reports hydrogen ion activity rather than concentration; in dilute solutions below about 0.1 M ionic strength the two agree closely enough for coursework.
Press the 10^x key (often shift + log), enter the pH with a minus sign, and press equals. For pH 4.2 that is 10^-4.2 = 6.31 × 10^-5 mol/L.
Pure water self-ionises slightly, and at 25 °C the product [H+][OH-] equals 1.0 × 10^-14. Because neutral water has equal amounts of each ion, both are the square root of that, 1.0 × 10^-7 mol/L, which is pH 7.
The digits after the decimal point in a pH are the significant figures of the concentration. A pH of 3.50 has two decimal places, so [H+] is reported as 3.2 × 10^-4 M with two significant figures. The integer part of the pH only sets the power of ten.
In water a free proton immediately bonds to a water molecule, so the species present is the hydronium ion H3O+. Chemists write [H+] and [H3O+] interchangeably; the numerical concentration is the same.
Yes. A strong acid above 1 mol/L, such as 2 M hydrochloric acid, gives a calculated pH of about -0.3. At those concentrations the activity departs from concentration, so the result here is an estimate.
The [H+] from a given pH does not depend on temperature, since it is a direct antilog. The pOH and [OH-] do, because Kw rises with temperature: at 50 °C pKw is about 13.26, so neutral water there has a pH near 6.63.
Using this tool in an assignment, article or report? Copy a citation, or a link for your website.
Tools Directory Editorial Team. (2026, October 17). pH to H+. Tools Directory. Retrieved October 27, 2026, from https://tools.milunir.com/chemistry/solutions-and-concentration/ph-to-h/
Tools Directory Editorial Team. “pH to H+.” Tools Directory, 17 Oct. 2026, https://tools.milunir.com/chemistry/solutions-and-concentration/ph-to-h/. Accessed 27 Oct. 2026.
Tools Directory Editorial Team. “pH to H+.” Tools Directory. Last modified October 17, 2026. Accessed October 27, 2026. https://tools.milunir.com/chemistry/solutions-and-concentration/ph-to-h/.
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