Which statement about pH and hydrogen ion concentration is true?

Which statement about pH and hydrogen ion concentration is true?

• A. pH is the inverse of [H+].
• B. pH is directly proportional to [H+].
• C. pH equals the same value as [H+].
• D. pH decreases as [H+] increases.

Answer: (D)

pH is defined as the negative base-10 logarithm of the hydrogen ion concentration: pH = -log10[H+]. This makes the relationship logarithmic, not a simple inverse or direct proportion. Because of the minus sign and the log, increasing [H+] lowers pH: a tenfold increase in [H+] reduces pH by one unit (pH = -log10[H+] means -log10(10×[H+]) = -1 - log10[H+], so pH drops by 1). Conversely, decreasing [H+] raises pH. So the statement that pH decreases as [H+] increases is the correct description of their relationship.

pH is defined as the negative base-10 logarithm of the hydrogen ion concentration: pH = -log10[H+]. This makes the relationship logarithmic, not a simple inverse or direct proportion. Because of the minus sign and the log, increasing [H+] lowers pH: a tenfold increase in [H+] reduces pH by one unit (pH = -log10[H+] means -log10(10×[H+]) = -1 - log10[H+], so pH drops by 1). Conversely, decreasing [H+] raises pH. So the statement that pH decreases as [H+] increases is the correct description of their relationship.