What is the standard formula for pH?

What is the standard formula for pH?

• A. pH = -log[H+]
• B. pH = log[H+]
• C. pH = -log[OH-]
• D. pH = 1/[H+]

Answer: (A)

The standard formula for pH uses the negative base-10 logarithm of the hydrogen ion concentration (or activity). This gives a convenient, logarithmic scale for acidity: higher hydrogen ion concentration means a lower pH, and lower hydrogen ion concentration means a higher pH. In dilute solutions, activity is close to concentration, so pH = -log10[H+]. For example, if the hydrogen ion concentration is 1×10^-3 M, the pH is 3. If [H+] is 1×10^-7 M, the pH is 7, which is neutral around room temperature. Why the other ideas don’t fit: using pH = log[H+] would assign higher pH to higher acidity, which is opposite of what pH represents. pH = -log[OH-] is actually the definition of pOH, not pH (and pH + pOH ≈ 14 at 25°C). pH = 1/[H+] is not a logarithmic measure and would not provide the proper, scalable sense of acidity across many orders of magnitude.

The standard formula for pH uses the negative base-10 logarithm of the hydrogen ion concentration (or activity). This gives a convenient, logarithmic scale for acidity: higher hydrogen ion concentration means a lower pH, and lower hydrogen ion concentration means a higher pH. In dilute solutions, activity is close to concentration, so pH = -log10[H+].

For example, if the hydrogen ion concentration is 1×10^-3 M, the pH is 3. If [H+] is 1×10^-7 M, the pH is 7, which is neutral around room temperature.

Why the other ideas don’t fit: using pH = log[H+] would assign higher pH to higher acidity, which is opposite of what pH represents. pH = -log[OH-] is actually the definition of pOH, not pH (and pH + pOH ≈ 14 at 25°C). pH = 1/[H+] is not a logarithmic measure and would not provide the proper, scalable sense of acidity across many orders of magnitude.