pH Calculator
Convert between pH, pOH, hydrogen ion concentration, and hydroxide ion concentration for classroom chemistry checks. The tool uses the common 25 C relationship pH + pOH = 14.
Relationship
pH + pOH = 14
Inputs
pH, pOH, H+, OH-
Scope
Educational
Live calculator
pH relationship input
pH
7
pOH
7
[H+]
1.000e-7 M
[OH-]
1.000e-7 M
Educational scope
Uses the classroom approximation pH + pOH = 14 at 25 C. This page is for chemistry learning and calculation checks, not medical, water-treatment, or laboratory safety decisions.
Convert between pH, pOH, and ion concentration
pH and pOH
Calculate both logarithmic values from one known pH, pOH, hydrogen, or hydroxide input.
Ion concentrations
Show [H+] and [OH-] in molarity so users can connect logarithms back to concentration.
Relationship checks
Keep pH + pOH and Kw assumptions visible for classroom problem checking.
pH formulas used on this page
The page uses the common classroom formulas for dilute aqueous solutions at 25 C. Formal pH definitions use hydrogen ion activity rather than simple concentration.
Working formulas
pH from hydrogen concentration
pH = -log10([H+])
Use molar concentration as the classroom approximation.
pOH from hydroxide concentration
pOH = -log10([OH-])
Hydroxide concentration is also entered in molarity.
Water relationship at 25 C
pH + pOH = 14
This approximation depends on temperature and solution assumptions.
Symbols
- pH - hydrogen ion logarithm
- The negative base-10 logarithm of hydrogen ion activity, approximated here from concentration.
- pOH - hydroxide ion logarithm
- The negative base-10 logarithm of hydroxide ion concentration in classroom examples.
- [H+] - hydrogen ion concentration
- The hydrogen ion concentration in moles per liter for simplified problems.
- [OH-] - hydroxide ion concentration
- The hydroxide ion concentration in moles per liter for simplified problems.
pH output with concentration and assumptions attached
Four related outputs
- One input produces pH, pOH, [H+], and [OH-] together.
- Concentration outputs use scientific notation so very small values remain readable.
- The result panel keeps the 25 C classroom assumption visible beside the answer.
- Copy and print controls support notes, worksheets, and teaching examples.
Careful chemistry wording
- The formula block explains that formal pH uses activity, while this tool uses concentration approximations for learning.
- FAQ answers warn that temperature, ionic strength, and real measurement conditions can change interpretation.
- The page avoids medical, environmental compliance, and lab-safety recommendations.
- Related links connect pH to concentration units and dilution arithmetic.
pH support for chemistry learning
Students
Check pH, pOH, and ion concentration problems with the full relationship visible.
Teachers
Create examples that show why one pH unit represents a tenfold concentration change.
Formula reviewers
Compare pH and pOH values without hiding the concentration values behind the logarithm.
How it works in three quick steps.
Choose the known value
Select pH, pOH, hydrogen ion concentration, or hydroxide ion concentration.
Enter the value
Type the known number using molarity for concentration inputs.
Review all four outputs
Read pH, pOH, [H+], and [OH-] together to check the logarithmic relationship.
Save or print a pH result
Copy the pH summary
Copy pH, pOH, [H+], [OH-], and the 25 C assumption in one note.
Print the calculation
Print the calculator, formulas, FAQ answers, and related chemistry links.
Compare inputs
Switch between pH, pOH, [H+], and [OH-] inputs to see how each value changes.
Why pH results need an assumption note
pH is one of the most familiar chemistry quantities, but the simple formula often hides important assumptions. In classrooms, students commonly use pH = -log10([H+]) and pH + pOH = 14 at 25 C. Those formulas are useful for learning logarithmic concentration relationships, but formal pH is defined through hydrogen ion activity. Toolarithm's pH Calculator keeps the simplified assumption visible so users can calculate quickly without mistaking the result for a measured laboratory value.
The calculator supports four input modes: pH, pOH, hydrogen ion concentration, and hydroxide ion concentration. Showing all four outputs together helps users see the inverse relationship between acidity and basicity, the tenfold nature of the pH scale, and the link between concentration and logarithms. The page deliberately avoids medical, environmental, or compliance advice because those contexts depend on validated measurement procedures and standards.
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