pH and pOH

High School

Definition

Logarithmic scales used to express how acidic or basic a solution is. pH measures the concentration of hydrogen ions and pOH the concentration of hydroxide ions; a low pH is acidic, a high pH is basic, and the two values always add up to 14 in water at 25°C.

Worked examples

\([\)H\(^+] = 1 \times 10^{-3} \) M\( \)→\( \)pH\( = -\log(10^{-3}) = 3\)

A hydrogen ion concentration of 0.001 M gives pH 3, which is acidic (less than 7).

\(\)pH\( = 11 \)→\( \)pOH\( = 14 - 11 = 3\)

When pH is 11 (basic), pOH must be 3 because the two always sum to 14 at 25°C.

\([\)OH\(^-] = 1 \times 10^{-5} \) M\( \)→\( \)pOH\( = 5, \) pH\( = 9\)

A hydroxide ion concentration of 0.00001 M gives pOH 5, so pH is 14 − 5 = 9 (basic).

Common mistakes

\(\)pH\( = 3\) is basic because 3 is a positive number → \(\)pH\( = 3\) is acidic; low pH means acidic, high pH means basic pH below 7 is acidic, above 7 is basic, and 7 is neutral.
\(\)pH\( + \)pOH\( = 14\) works at any temperature → \(\)pH\( + \)pOH\( = 14\) only at 25°C in water The sum changes with temperature because water's ionization constant varies.
\(\)pH\( = \log[\)H\(^+]\) → \(\)pH\( = -\log[\)H\(^+]\) The negative sign is essential; pH is the negative logarithm of hydrogen ion concentration.

Where you'll use it next

You'll use pH and pOH when studying acid-base equilibria, buffer solutions, and titration curves in chemistry, and when interpreting water quality, blood pH, and soil chemistry in biology and environmental science.

Found in 1 StudyPug lesson

Mastering pH and pOH: Essential Chemistry Concepts

GCSE Chemistry

Dive into the world of pH and pOH with our engaging video introduction. Understand acidity, basicity, and their roles in chemical processes. Build a strong foundation for advanced chemistry studies and real-world applications.

10th Grade10th