Which statement about pH and hydrogen ion concentration is true?

Which statement about pH and hydrogen ion concentration is true?

• A. pH is the reciprocal of hydrogen ion concentration.
• B. A smaller pH corresponds to a smaller hydrogen ion concentration.
• C. pH is inversely related to hydrogen ion concentration logarithmically.
• D. pH is equal to the negative log of hydroxide concentration.

Answer: (C)

The main idea is how pH relates to hydrogen ion concentration. pH is defined as the negative base-10 logarithm of [H+]: pH = -log10[H+]. This makes the relationship logarithmic and inverse: as [H+] increases, pH decreases, and as [H+] decreases, pH increases. Because of the log scale, a tenfold rise in [H+] lowers pH by 1 unit, while a tenfold drop raises pH by 1 unit. This is why pH is inversely related to hydrogen ion concentration on a logarithmic scale. The other statements don’t fit that relationship: pH is not the reciprocal of [H+], smaller pH means larger [H+], and pH is not the negative log of hydroxide concentration (that would involve pOH).

The main idea is how pH relates to hydrogen ion concentration. pH is defined as the negative base-10 logarithm of [H+]: pH = -log10[H+]. This makes the relationship logarithmic and inverse: as [H+] increases, pH decreases, and as [H+] decreases, pH increases. Because of the log scale, a tenfold rise in [H+] lowers pH by 1 unit, while a tenfold drop raises pH by 1 unit. This is why pH is inversely related to hydrogen ion concentration on a logarithmic scale. The other statements don’t fit that relationship: pH is not the reciprocal of [H+], smaller pH means larger [H+], and pH is not the negative log of hydroxide concentration (that would involve pOH).