Calculate The Ph of Pure Water at 25 Degrees Celsius

Pure water at 25°C has a well-defined pH value that serves as a reference point in chemistry. This calculator helps you determine this fundamental value using the standard pH formula.

What is pH?

The pH scale measures how acidic or basic a solution is. It ranges from 0 to 14, where:

pH 0-6.9: Acidic
pH 7: Neutral (like pure water)
pH 7.1-14: Basic (alkaline)

The pH scale is logarithmic, meaning each whole number change represents a tenfold difference in hydrogen ion concentration.

pH of Pure Water

Pure water at 25°C has a pH of exactly 7.000. This is because pure water is neutral, containing equal concentrations of hydrogen (H⁺) and hydroxide (OH^-) ions.

At temperatures other than 25°C, the pH of pure water changes slightly. For example, at 0°C it's 7.47 and at 100°C it's 6.14.

Calculation Method

The pH of pure water can be calculated using the following formula:

pH = -log₁₀[H⁺]

Where H⁺ is the concentration of hydrogen ions in moles per liter (mol/L). For pure water at 25°C, the concentration of H⁺ ions is known to be 1.0 × 10^-7 mol/L.

Assumptions

Standard temperature of 25°C (298.15 K)
Pure water with no dissolved solutes
Ion product of water (Kw) = 1.0 × 10^-14 at 25°C

Example Calculation

Let's calculate the pH of pure water at 25°C using the known H⁺ concentration:

pH = -log₁₀[1.0 × 10⁻⁷] pH = -(-7) pH = 7.000

This confirms that pure water at 25°C has a pH of exactly 7.000.

FAQ

Why is the pH of pure water exactly 7?

Pure water is neutral because it contains equal concentrations of hydrogen (H⁺) and hydroxide (OH⁻) ions, which cancel each other out on the pH scale.

Does the pH of pure water change with temperature?

Yes, the pH of pure water changes slightly with temperature. At 0°C it's 7.47 and at 100°C it's 6.14, with 7.000 at 25°C.

What is the ion product of water (Kw)?

The ion product of water (Kw) is the product of the concentrations of H⁺ and OH⁻ ions in pure water. At 25°C, Kw = 1.0 × 10⁻¹⁴.