pH / Buffer Calculator
Calculate pH for strong acids/bases or buffer solutions.
Result pH
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How to Calculate pH
Calculating pH from Concentration
The pH of a solution is a measure of its acidity or alkalinity. For strong acids and bases, it can be calculated directly from their concentration.
For a Strong Acid:
The formula is:
pH = -log₁₀[H⁺]
Where [H⁺] is the concentration of hydrogen ions, which is equal to the concentration of the strong acid.
Example: For a 0.1 M HCl solution, the pH is -log₁₀(0.1) = 1.
For a Strong Base:
First, calculate pOH:
pOH = -log₁₀[OH⁻]
Where [OH⁻] is the concentration of hydroxide ions, equal to the concentration of the strong base.
Then, convert pOH to pH:
pH = 14 - pOH
Example: For a 0.1 M NaOH solution, pOH = -log₁₀(0.1) = 1, so pH = 14 - 1 = 13.
Calculating Buffer pH (Henderson-Hasselbalch Equation)
A buffer solution resists changes in pH. Its pH can be calculated using the Henderson-Hasselbalch equation, which relates the pH of a solution to the pKa of the weak acid and the ratio of the concentrations of the conjugate base and weak acid.
pH = pKa + log₁₀( [A⁻] / [HA] )
Where:
- pKa is the acid dissociation constant of the weak acid.
- [A⁻] is the concentration of the conjugate base.
- [HA] is the concentration of the weak acid.
Example Calculation
Consider a buffer made from 0.1 M acetic acid ([HA]) and 0.1 M sodium acetate ([A⁻]). The pKa of acetic acid is 4.76.
pH = 4.76 + log₁₀( 0.1 / 0.1 )
pH = 4.76 + log₁₀(1) = 4.76
The pH of the buffer solution is 4.76.
Quick FAQs
- What is the pH scale?
- The pH scale ranges from 0 to 14. 7 is neutral, below 7 is acidic, and above 7 is basic (alkaline).
- Can pH be negative?
- Yes, for very concentrated strong acids, the pH can drop below zero.
- When does the Henderson-Hasselbalch equation fail?
- It is an approximation. It works best when the concentrations are high and the pH is near the pKa. It fails for very dilute solutions or very strong/weak acids.