Question

What is the measure of hydroxide ion concentration?
A) $\text{ pH }$
B) $\text{ pOH }$
C) $\text{ }{{\text{K}}_{\text{a}}}\text{ }$
D) Conjugate acid
E) Conjugate base

Answer 1

Every aqueous solution, whether acidic or alkaline or neutral, contains both ${{\text{H}}^{\text{+}}}$ and $\text{ O}{{\text{H}}^{-}}\text{ }$ ions. One scale which is very popular for chemical concentration depends on the p-function. This quantities ‘X’ of interest is written as:
$\text{ pX = }-\text{log X = log }\dfrac{1}{\text{X}}\text{ }$

Complete Solution :
- Hydroxide ion concentration is used to express the basicity of the solution. The concentration of the hydronium ion $\text{ }{{\text{H}}_{\text{3}}}{{\text{O}}^{\text{+}}}\text{ }$ or the hydrogen ion ${{\text{H}}^{\text{+}}}$ and hydroxide ion $\text{ O}{{\text{H}}^{-}}\text{ }$ in the aqueous solution. We are well familiar with the term $\text{ pH }$ . The $\text{ pH }$ is used to represent hydrogen ion concentration. Similarly the hydroxide ion concentration in a solution can be expressed in terms of $\text{ pOH }$ scale.

- Chemical concentrations are expressed in the logarithmic scale. According to which the concentrations are based on the p-function .Let consider a chemical quantity ‘X’ of the interest and its logarithmic value is written as a ‘log’ as a base -10. The chemical quantity is written as,
$\text{ pX = }-\text{log X = log }\dfrac{1}{\text{X}}\text{ }$

- Thus hydroxide ion concentration is expressed as a p-function. The $\text{ pOH }$ of a solution is defined as the negative logarithmic of the hydrogen ion concentration .it is represented as follows,
$\text{ pOH = }-\text{log }\left[ \text{O}{{\text{H}}^{-}} \right]\text{ }$
Where , $\left[ \text{O}{{\text{H}}^{-}} \right]$ is a hydroxide ion concentration .

- We can measure the acidity or basicity of the acid and base using the $\text{ pH }$ and $\text{ pOH }$ scale. The $\text{ pOH }$ scale is used to measure the hydroxide ion concentration in the solution. The$\text{ pH }$ and $\text{ pOH }$ scale goes side by side. When we are measuring the acid the $\text{ pH }$ value is less than 7 but $\text{ pOH }$ it will be greater than the 7.
- However for bases, the $\text{ pH }$ value would be more than 7, but $\text{ pOH }$ the value would be less than the 7. Dissociation constant $\text{ }{{\text{K}}_{\text{a}}}\text{ }$ is the measure of the extent of dissociation of the acid in the solution. Thus it cannot be used to determine the hydroxide ion concentration from the solution.
Thus, the hydroxide ion concentration is measured in terms of $\text{ pOH }$.
So, the correct answer is “Option B”.

Note: Note that, the hydroxide ion concentration can be also written as the antilog of $\text{ pOH }$. It is written as, $\text{ }\left[ \text{O}{{\text{H}}^{-}} \right]\text{ = 1}{{\text{0}}^{-\text{pOH }}}\text{ }$ .In a neutral solution, the value of $\text{ pH }$ values is equal to $\text{ pOH }$. The logarithmic values of base 10 are of very much importance. The logarithmic scale magnifies the concentration scale by the power of 10.