Frequently Asked Questions

Common questions about pH calculations and acid-base chemistry

pH stands for "potential of Hydrogen" and measures the acidity or alkalinity of a solution. It's calculated using the formula:

pH = -log₁₀[H⁺]

Where [H⁺] is the concentration of hydrogen ions in moles per litre (M). The pH scale ranges from 0 to 14, with 7 being neutral, values below 7 acidic, and values above 7 basic (alkaline).

pH measures hydrogen ion concentration [H⁺], while pOH measures hydroxide ion concentration [OH⁻]:

  • pH = -log₁₀[H⁺]
  • pOH = -log₁₀[OH⁻]
  • pH + pOH = 14 (at 25°C)

This relationship allows you to calculate one value if you know the other.

Strong acids/bases completely dissociate in water:

  • Strong acids: HCl, H₂SO₄, HNO₃, HBr, HI, HClO₄
  • Strong bases: NaOH, KOH, Ca(OH)₂, Ba(OH)₂
  • pH calculation is straightforward: pH = -log₁₀(concentration)

Weak acids/bases only partially dissociate:

  • Examples: CH₃COOH (acetic acid), NH₃ (ammonia)
  • Require Ka or Kb values for accurate pH calculation
  • Use equilibrium expressions for calculation

Ka (acid dissociation constant) measures the strength of a weak acid:

Ka = [H⁺][A⁻]/[HA]

Kb (base dissociation constant) measures the strength of a weak base:

Kb = [BH⁺][OH⁻]/[B]

Larger Ka or Kb values indicate stronger acids or bases. These constants are essential for calculating the pH of weak acid/base solutions.

Temperature affects the water dissociation constant (Kw):

  • At 25°C: Kw = 1.0 × 10⁻¹⁴
  • Higher temperatures increase Kw
  • This affects the neutral pH (which isn't always exactly 7)
  • Our calculator accounts for temperature variations

A neutral solution has equal concentrations of H⁺ and OH⁻ ions:

  • At 25°C: pH = 7.00 is neutral
  • [H⁺] = [OH⁻] = 1.0 × 10⁻⁷ M
  • Pure water at 25°C is the classic example
  • The neutral pH changes slightly with temperature

Our calculations are scientifically accurate for educational and most practical purposes:

  • Strong acids/bases: Highly accurate assuming complete dissociation
  • Weak acids/bases: Uses standard approximations valid for most concentrations
  • Temperature effects are included in calculations
  • For extremely dilute solutions or complex systems, consult advanced chemistry resources

PHCalc.Com is designed as an educational tool:

  • Perfect for checking your manual calculations
  • Shows step-by-step working to help you learn
  • Always verify with your instructor's guidelines for calculator use
  • Use it to understand concepts, not replace learning the fundamentals

Always follow proper safety protocols:

  • Wear safety goggles, gloves, and lab coats
  • Work in well-ventilated areas or fume hoods
  • Never add water to concentrated acid - always add acid to water
  • Have neutralising agents and eyewash stations available
  • Know the location of safety showers and first aid equipment
  • Never taste or smell chemicals directly

Reliable sources for acid-base constants include:

  • CRC Handbook of Chemistry and Physics
  • NIST Chemistry WebBook
  • Chemistry LibreTexts
  • Your chemistry textbook's appendix
  • Our Acid & Base Database for common compounds