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TL;DR

To calculate pH, take the negative logarithm of the hydronium ion concentration. The pOH can then be found by subtracting the pH from 14. Conversely, to find pH from a known pOH, simply subtract the pOH from 14.

What Do pH and pOH Measure?

When a strong acid dissolves in water, it releases hydronium ions (H₃O⁺). This occurs because the acid’s proton separates from the acid molecule and associates with a water molecule. For example, hydrochloric acid (HCl) behaves as follows:

The pH scale indicates the concentration of H₃O⁺, while the pOH scale reflects the concentration of hydroxide ions (OH^-). These two values are complementary and offer a complete picture of a solution’s acidity or basicity.

How to Calculate pH and pOH

The “p” in pH and pOH stands for “–log.” Thus:

• pH = –log[H₃O⁺]
• pOH = –log[OH^-]

Strong acids and bases dissociate completely in water. If you know the molarity of a strong acid, you immediately know the [H₃O⁺] concentration. For instance, a 0.001 M HCl solution yields [H₃O⁺] = 0.001 M, so:

To find the corresponding pOH, use the intrinsic relationship for water at 25 °C:

Thus, if pH = 3, then pOH = 11. Conversely, for a strong base with pH = 13, pOH = 1.

Identifying Acidic and Basic Solutions

Interpretation of the pH scale is straightforward:

• pH < 7: acidic (high H₃O⁺ concentration)
• pH = 7: neutral (equal H₃O⁺ and OH^- concentrations)
• pH > 7: basic (low H₃O⁺ concentration)

These guidelines allow you to quickly assess whether a solution is acidic, neutral, or basic based on its pH or pOH value.