pH Calculator
Compute pH from hydrogen ion concentration, from pOH, or from a weak acid/base concentration with Ka or Kb. Returns pH, pOH, [H+], [OH-], and an acidic/neutral/basic verdict.
Compute pH
Use scientific notation like 1e-7 for 10^-7 mol/L.
pH
4
acidic
pOH
10
[H+] (mol/L)
1.000e-4
[OH-] (mol/L)
1.000e-10
Frequently Asked Questions about the pH Calculator
Why is the pH scale logarithmic?
pH is defined as -log10([H+]), so each whole pH unit is a 10x change in hydrogen ion concentration. A pH 4 solution has 1000 times more H+ ions than a pH 7 solution, which is why small pH shifts matter so much.
What is the difference between a weak and a strong acid?
A strong acid (HCl, HNO3) dissociates fully in water, so [H+] equals the acid concentration. A weak acid (acetic acid, citric acid) only partially dissociates and needs the equilibrium constant Ka.
Why does pH plus pOH always equal 14?
Water self-ionizes with Kw = [H+][OH-] = 1.0e-14 at 25 degrees C. Taking the negative log of both sides gives pH + pOH = 14. The relationship only holds exactly at 25 C.
What are typical pH values of everyday substances?
Stomach acid near pH 1.5, lemon juice near 2, black coffee near 5, pure water 7, blood 7.4, baking soda solution 9, household ammonia 11, oven cleaner near 13. Below 7 is acidic, above 7 is basic.
When does the sqrt(Ka * C) approximation break down?
It assumes the dissociated fraction is small (under about 5 percent). For very dilute solutions or moderately strong weak acids, you need the full quadratic: x^2 + Ka*x - Ka*C = 0.