pH is defined as

pH is defined as

• A. log[H+]
• B. -log[H+]
• C. log[H+]^2
• D. pOH

Answer: (B)

The idea being tested is how pH translates hydrogen ion concentration into a compact scale using a negative logarithm. pH is defined as pH = -log10[H+]. The negative sign is essential because it makes more acidic solutions (higher [H+]) have lower pH values, while more basic solutions (lower [H+]) have higher pH values. The base-10 logarithm also compresses the wide range of possible [H+] in aqueous solutions into a practical scale, typically around 0 to 14 at 25°C. If you used log10[H+], acidic solutions would appear with higher numbers, which would invert the intuitive sense of acidity. If you took log10[H+] squared, you’d get 2 log10[H+], which isn’t pH at all. pOH, defined as -log10[OH-], is a related quantity but not pH, though pH and pOH sum to about 14 at room temperature.

The idea being tested is how pH translates hydrogen ion concentration into a compact scale using a negative logarithm. pH is defined as pH = -log10[H+]. The negative sign is essential because it makes more acidic solutions (higher [H+]) have lower pH values, while more basic solutions (lower [H+]) have higher pH values. The base-10 logarithm also compresses the wide range of possible [H+] in aqueous solutions into a practical scale, typically around 0 to 14 at 25°C.

If you used log10[H+], acidic solutions would appear with higher numbers, which would invert the intuitive sense of acidity. If you took log10[H+] squared, you’d get 2 log10[H+], which isn’t pH at all. pOH, defined as -log10[OH-], is a related quantity but not pH, though pH and pOH sum to about 14 at room temperature.