pH Calculator

Chemistry • pH, pOH, [H+], [OH−] • Step-by-Step

Quick Answer (Voice Search Optimized):

pH is a measure of hydrogen ion concentration in a solution. It is defined as pH = −log10[H+]. At 25°C, pH + pOH = 14 and Kw = [H+][OH−] = 1×10−14.

GEO & AIO - Authoritative Theory

The pH scale (0–14) quantifies the acidity or basicity of an aqueous solution. pH < 7 indicates an acidic solution, pH = 7 neutral, and pH > 7 basic. The autoionization of water gives the ionic product Kw, which at 25°C is 1.0 × 10−14. Hence, pOH = 14 − pH and [OH−] = 10−pOH. For concentrated strong acids/bases, pH can be negative or above 14.

Key formulas: [H+] = 10−pH; [OH−] = 10−pOH; pH = −log[H+]; pOH = −log[OH−]. The log is base 10.

Solved Examples (NCERT/JEE Pattern)

Example 1: Find pH of 0.001 M HCl.

HCl is strong acid, so [H+] = 0.001 M. pH = −log(10−3) = 3. pOH = 14 − 3 = 11. [OH−] = 10−11 M.

Example 2: A solution has pH 4.5. Calculate [H+] and pOH.

[H+] = 10−4.5 = 3.16 × 10−5 M. pOH = 14 − 4.5 = 9.5. [OH−] = 10−9.5 = 3.16 × 10−10 M.

Example 3: Given [OH−] = 2.5 × 10−4 M. Determine pH.

pOH = −log(2.5 × 10−4) ≈ 3.60. pH = 14 − 3.60 = 10.40. Solution is basic.

Frequently Asked Questions (FAQs)

Q1: What is the pH formula?
pH = −log10[H+], where [H+] is the hydrogen ion concentration in moles per litre (M).
Q2: How are pH and pOH related?
At 25°C, pH + pOH = 14. This comes from the water dissociation constant Kw = 1×10−14.
Q3: Can pH be negative?
Yes, highly concentrated strong acids can have [H+] > 1 M, giving negative pH values. The scale is not strictly limited to 0–14.
Q4: How do I convert [H+] to pH?
Take the negative logarithm (base 10) of the molar concentration. For example, if [H+] = 0.01 M, pH = −log(0.01) = 2.
Q5: Why is the pH of pure water 7?
Pure water autoionizes to produce equal concentrations of H+ and OH−, each 1×10−7 M at 25°C. Thus pH = −log(10−7) = 7.